surnames.ie

Irish surname · 11th most common in 1911

O'Reilly

Also spelt Reilly, Rielly, ORielly

O'Reilly and Reilly come from the Irish Ó Raghallaigh, 'descendant of Raghallach'. The O'Reillys were kings of East Breifne, roughly today's County Cavan, from the 13th century until the early 1600s, and they were close kin of the O'Rourkes of West Breifne.

Irish Ó Raghallaigh, Ó Raghailligh · Ó RaghallaighIrish Ó Raghaill
1901 census
24,874
#11 · 56 per 10,000
1911 census
24,389
#11 · 56 per 10,000
1926 census
21,842
#9 · 74 per 10,000
Most numerous
Cavan
Highest share: Cavan

Language & meaning

Meaning & Irish forms

Gaelic Gaelic Irish: a ruling family of the Uí Briúin Bréifne, kings of East Breifne (County Cavan) and of the same stock as the O'Rourkes.[1][2][3]

Ó Raghallaigh, Ó Raghailligh
Modern spelling: Ó Raghallaigh
“descendant of Raghallach”[1][2]
A daughter is Ní Raghallaigh and a wife Bean Uí Raghallaigh. Woulfe records dialect pronunciations such as Ó Rághallaigh, Ó Raighilligh and Ó Raoghallaigh.
Ó Raghaill
“a shortened form of Ó Raghallaigh”[4]
This short form gives O'Rahill, Rahill, Reihill, Ryle and similar names.

The name is built on the personal name Raghallach, usually explained as “not established by the sources”. None of the sources consulted gives a reliable meaning for the name. Wikipedia says Raghallach was a chieftain of the early 11th century and repeats an uncited tradition that he died at Clontarf in 1014.[2][3]

Pronunciation

O'Reilly: oh-RY-lee (Reilly: RY-lee)

Place

Where the O'Reillys lived

How common the name was in each county, as a share of the population, in the three surviving full censuses of Ireland. Use the buttons to compare years.

Donegal: 5.8 per 10,000 (98 people) Waterford: 18.7 per 10,000 (154 people) Dublin: 76.8 per 10,000 (3,691 people) Kilkenny: 24.8 per 10,000 (186 people) Carlow: 14.6 per 10,000 (53 people) Limerick: 15.6 per 10,000 (224 people) Louth: 100.8 per 10,000 (635 people) Tipperary: 17.8 per 10,000 (272 people) Roscommon: 32.5 per 10,000 (305 people) Meath: 305.6 per 10,000 (1,993 people) Wicklow: 84 per 10,000 (511 people) Cavan: 638.8 per 10,000 (5,835 people) Cork: 24.9 per 10,000 (979 people) Clare: 7.8 per 10,000 (81 people) Galway: 49.4 per 10,000 (901 people) Mayo: 67.5 per 10,000 (1,297 people) Sligo: 24.6 per 10,000 (195 people) Leitrim: 111.8 per 10,000 (705 people) Offaly: 29 per 10,000 (165 people) Laois: 33.8 per 10,000 (185 people) Kildare: 49.9 per 10,000 (334 people) Kerry: 15 per 10,000 (239 people) Monaghan: 52 per 10,000 (372 people) Wexford: 26.9 per 10,000 (275 people) Longford: 358 per 10,000 (1,569 people) Westmeath: 131.6 per 10,000 (792 people) Tyrone: 12.7 per 10,000 (182 people) Antrim: 14.5 per 10,000 (692 people) Armagh: 18.7 per 10,000 (224 people) Fermanagh: 114.2 per 10,000 (693 people) Londonderry: 16.2 per 10,000 (226 people) Down: 10.7 per 10,000 (326 people)
fewerup to 650 per 10,000

Share of each county's census population. 1926 covers the 26 counties of the Irish Free State only.

Highest concentration · 1911

Cavan

639 per 10,000

5,835 O'Reillys · 11.5× the national share

Highest share, 1911

  1. Cavan 639
  2. Longford 358
  3. Meath 306
  4. Westmeath 132
  5. Fermanagh 114
  6. Leitrim 112
  7. Louth 101
  8. Wicklow 84

Per 10,000 residents. A concentration index above 1 means the name was commoner there than nationally.

County figures as a table
County190119111926Per 10k 1901Per 10k 1911Per 10k 1926
Antrim674692—1515—
Armagh244224—2019—
Carlow705381191524
Cavan6,3325,8355,297650639644
Clare1188111711812
Cork966979986242527
Donegal13598131869
Down246326—911—
Dublin3,4303,6914,194787784
Fermanagh665693—103114—
Galway950901935494955
Kerry342239281211519
Kildare326334328535057
Kilkenny151186155202522
Laois212185153373430
Leitrim784705610113112108
Limerick223224232151617
Londonderry252226—1716—
Longford1,7491,5691,361374358342
Louth62363568896101110
Mayo1,2561,2971,290636875
Meath2,0841,9932,015297306322
Monaghan438372479595274
Offaly110165160182931
Roscommon249305257253331
Sligo176195221222531
Tipperary234272219151816
Tyrone178182—1213—
Waterford151154180171923
Westmeath745792775120132137
Wexford248275253242727
Wicklow513511444868478

Local strongholds in 1911

District Electoral Divisions (300+ residents) where O'Reillys made up the largest share of the population. In Drumlumman, Co. Cavan, about one resident in 5 was a O'Reilly.

DEDCountyO'ReillysShare
DrumlummanCavan178 19.6%
LoughdawanCavan161 19.5%
AshyouleFermanagh158 17.5%
MoyneLongford158 16.8%
KilcogyCavan147 15.4%
ColumbkilleLongford184 15.2%
CreevyLongford108 15.1%
CorrCavan76 15.0%

Origins

Septs & territories

Many Irish surnames were adopted independently by unrelated families. These are the distinct O'Reilly lineages described in the sources.

A surname can point you towards a place. On its own, it cannot establish a pedigree.

Ó Raghallaigh of Breifne O'Reilly (East Breifne)

Bréifne Uí Raghallaigh, at first around Lough Ramor and later all of County Cavan · Cavan, Meath, Westmeath

The family emerged from the Muintir Maelmórdha, a branch of the Uí Briúin, and took the O'Reilly name from Raghallach in the early 11th century. After Breifne split in 1256, it ruled East Breifne. Woulfe says that during the 13th and 14th centuries its power extended over all of Cavan and into Meath and Westmeath, and that at times it ruled all of Breifne.[1][3]

The name over time

History

Origins in Breifne

The O'Reillys belonged to the Uí Briúin Bréifne and shared their ancestry with the O'Rourkes. Wikipedia traces them to a branch called Muintir Maelmórdha, named after a 9th-century chief, which took the surname Ó Raghallaigh from Raghallach in the early 11th century. They first appear in the annals in 1126, ruling Machaire Gailenga around Lough Ramor as vassals of the O'Rourkes. The tradition that Raghallach himself died at Clontarf in 1014 is repeated on Wikipedia without a supporting citation.[3][2][1]

Kings of East Breifne

After a long struggle with the O'Rourkes, Breifne split in 1256. The O'Reillys held the eastern half, essentially modern Cavan, and the O'Rourkes kept the west, essentially Leitrim. Belturbet and then Cavan served as the O'Reilly capital, and their kings lived on Tullymongan Hill above Cavan town from the late 13th century. They were inaugurated on Shantemon Hill nearby. In 1233 the family had already taken Cloughoughter Castle, a Norman island fortress on Lough Oughter, and they kept it for most of the Middle Ages.[3][5][6]

Coinage and the Church

East Breifne was unusual among Gaelic lordships in having its own coins. 'O'Reilly's money' was struck at Cavan in imitation of English coins of the early 15th century. Acts of Parliament in 1447 and 1456 banned 'Reilly's silver', and Wikipedia notes that the area was a centre for counterfeiting. The family were also important patrons of the Church. King Giolla Íosa Ruadh founded the Franciscan friary at Cavan in the early 14th century, and the O'Reillys supplied many bishops to Kilmore. Woulfe notes that five of the name became Primates of Armagh.[3][1]

Shiring, war and plantation

In 1579 East Breifne was made into County Cavan. The 1584 composition agreed by John O'Reilly with Lord Deputy Perrot divided the lordship and abolished the chieftaincy, though it was never fully carried out. Woulfe says the clan kept its independence until the time of James I. The kingdom ended in 1607, after the Nine Years' War, and much of Cavan was planted with settlers. In 1641 Philip O'Reilly raised a large army in Cavan, and Cloughoughter held out until 1653. Woulfe says the family kept considerable property until the Cromwellian confiscations.[3][5][7][1]

A name of many spellings

Woulfe observed that O'Reilly had become one of the most common Irish names. He also noted that because Irish speakers pronounced it in so many dialect forms, it was written in English in many different ways. His list runs from O Reyly and O Riellie in early records to Reilly, Rielly, Really, Reely and Riley. The shortened Irish form Ó Raghaill produced a further set, including O'Rahill, Reihill and Ryle. Many O'Reillys also served abroad, notably Alejandro O'Reilly in Spain and Andreas O'Reilly in Austria.[1][4][8][9]

Places linked to the name

  • Cloughoughter Castle (Cloch Locha Uachtair), Co. Cavan — An island castle in Lough Oughter, held by the O'Reillys from 1233 and in 1641–53 by Philip O'Reilly.[5]
  • Tullymongan Hill, Co. Cavan — Residence of the O'Reilly kings from the late 13th century into the 17th.[3]
  • Shantemon Hill (Seantóman), Co. Cavan — Inauguration site of the O'Reilly kings, about 5 km from Cavan town.[3]
  • Cavan town (An Cabhán), Co. Cavan — The O'Reilly capital from around 1300, where the family founded a Franciscan friary and minted coins.[3]
  • Lough Ramor, Co. Cavan — Machaire Gailenga, around this lake, is where the O'Reillys first appear in the annals in 1126.[3]

Everyday lives

The O'Reillys in the census

Who the O'Reillys were in the 1901 and 1911 censuses (and 1926 where recorded), set against everyone in Ireland in the same census.

O'Reillys All of Ireland Ireland's figure marked on the O'Reilly bar

Religion, 1911

Catholic +22.9 points vs Ireland
O'Reillys 96.5%
All of Ireland 73.6%
Church of Ireland −11.0 points vs Ireland
O'Reillys 2.2%
All of Ireland 13.2%
Presbyterian −9.2 points vs Ireland
O'Reillys 0.9%
All of Ireland 10.1%
Methodist & other −2.8 points vs Ireland
O'Reillys 0.3%
All of Ireland 3.1%

Irish language

Spoke Irish, 1901 −5.4 points vs Ireland
O'Reillys 9.9%
All of Ireland 15.3%
Spoke Irish, 1911 −4.2 points vs Ireland
O'Reillys 9.3%
All of Ireland 13.5%
Spoke Irish, 1926 · 26 counties −2.7 points vs Ireland
O'Reillys 10.8%
All of Ireland 13.5%

104 O'Reillys spoke Irish only in 1901.

Reading & writing

Could read and write, 1901 · aged 15+ −1.1 points vs Ireland
O'Reillys 79.3%
All of Ireland 80.4%
Could read and write, 1911 · aged 15+ −1.5 points vs Ireland
O'Reillys 84.3%
All of Ireland 85.8%

Family life, 1911

Household size · 5,282 households lower than Ireland
O'Reillys 4.61
All of Ireland 4.83
Children born per mother · women 45–59 higher than Ireland
O'Reillys 6.56
All of Ireland 6.46
Children still living +1.1 points vs Ireland
O'Reillys 80.9%
All of Ireland 79.8%

Commonest occupations

Adults aged 15+, 1901 and 1911 combined.

Farmer5,085
Farmer's son2,531
Farmer's daughter1,236
Agricultural labourer951
General labourer898
Labourer737
Farm servant678
House keeper508
Farm labourer502
Domestic servant423

Born outside Ireland

1.3% of O'Reillys in 1911 gave a birthplace outside Ireland.

England171
Scotland71
United States69
India7
Australia & NZ2

Median age: 24 in 1901, 26 in 1911.

Given names

First names among the O'Reillys

Men and boys

  1. John4,968
  2. Patrick4,468
  3. James3,552
  4. Thomas2,797
  5. Michael2,496
  6. William1,289
  7. Peter1,102
  8. Edward1,090
  9. Bernard950
  10. Joseph850

Women and girls

  1. Mary5,384
  2. Bridget2,983
  3. Margaret1,781
  4. Ellen1,422
  5. Annie1,354
  6. Kate1,337
  7. Catherine1,321
  8. Anne1,243
  9. Maggie967
  10. Elizabeth903

All three censuses combined. Top five in 1926: John, Patrick, James, Thomas, Michael; Mary, Bridget, Margaret, Annie, Ellen.

Spellings

Spellings

This page combines the spellings below, which census enumerators and families used for the same name.

Spelling190119111926Records
Reilly19,70118,48514,129Search
O'Reilly3,4484,5976,927Search
Rielly1,4831,004662Search
ORielly242303124Search

Related forms

  • Reilly — O'Reilly without the O' prefix, and the commonest variant.[1][2]
  • Riley, O'Riley, Rielly — Further phonetic spellings of Ó Raghallaigh. Riley can also be an unrelated English surname.[1][2]
  • Really, Realy, Reely — Spellings that reflect the dialect pronunciation Ó Raoghallaigh.[1][10]
  • O'Rahill, Rahill, Reihill, Ryle — From Ó Raghaill, a shortened form of Ó Raghallaigh.[4]

Diaspora

The O'Reillys abroad

Emigration carried the name worldwide. In the 2010 United States census:

SpellingPeopleUS rank
Reilly40,884845th
O'Reilly12,0742970th
Rielly93926086th
ORielly20390853rd

Source: US Census Bureau, 2010 surname file (public domain).

Heraldry

Is there a O'Reilly coat of arms?

Arms belonged to particular families and individuals, not to everyone named O'Reilly. Wikipedia's article on East Breifne shows a banner and coat of arms for the O'Reilly rulers with the motto 'Fortitudine et prudentia' ('with fortitude and prudence'), but it does not describe the design. A defaced form of the O'Reilly crest appears in the arms of County Cavan.[3][2]

Websites selling a single “O'Reilly family crest” are selling a design, not an inheritance. Arms in Ireland belonged to individuals and their heirs, and the Chief Herald of Ireland still grants arms that way today.

People

Notable people named O'Reilly

  • Hugh O'Reilly (c. 1580–1653)
    Bishop of Kilmore, then Catholic Archbishop of Armagh from 1628 to 1653.
  • Edmund O'Reilly (1598–1669)
    Catholic Archbishop of Armagh and Primate of All Ireland, 1657–1669; descended from the East Breifne chiefs.
  • Philip O'Reilly (1599–1655)
    MP for County Cavan and a leader of the 1641 rebellion.
  • Alejandro O'Reilly (1723–1794)
    Irish-born Spanish general, governor of Louisiana in 1769.
  • Andreas O'Reilly von Ballinlough (1742–1832)
    Irish-born Austrian general who served from the Seven Years' War to the Napoleonic Wars.
  • Thomas Devin Reilly (1824–1854)
    Young Ireland revolutionary and journalist.
  • Myles O'Reilly (1825–1880)
    Balbriggan-born Catholic soldier, MP and publicist.
  • Peter O'Reilly (1827–1905)
    Irish-born official in colonial British Columbia, who headed its Indian reserve commission.
  • John Boyle O'Reilly (1844–1890)
    Meath-born Fenian poet, transported to Western Australia, who escaped and edited The Pilot in Boston.
  • Leonora O'Reilly (1870–1927)
    American feminist, suffragist and trade union organiser.
  • Tony O'Reilly (1936–2024)
    Rugby international, then head of H.J. Heinz and Independent News & Media.
  • Tim O'Reilly (born 1954)
    Irish-American publisher, founder of O'Reilly Media, who popularised 'open source' and 'Web 2.0'.

Questions

Questions about the O'Reilly name

Is O'Reilly an Irish surname?

Yes. O'Reilly is the anglicised form of Ó Raghallaigh, 'descendant of Raghallach'. The family belonged to the Uí Briúin of Breifne and ruled East Breifne, more or less today's County Cavan, from 1256 until the early 17th century. Woulfe noted that it had become one of the most common surnames in Ireland.[1][2][3]

What is O'Reilly in Irish?

The Irish form is Ó Raghallaigh. A woman born into the family is Ní Raghallaigh, and a married woman may use Bean Uí Raghallaigh or Uí Raghallaigh. Woulfe also records several dialect pronunciations, including Ó Raighilligh and Ó Raoghallaigh, and a short form, Ó Raghaill, which produced names such as O'Rahill and Reihill.[2][1][4]

Are Reilly and O'Reilly the same name?

Yes. Both come from Ó Raghallaigh, and Reilly is simply the form without the O'. Woulfe lists Reilly, O'Reilly, Rielly, Really, Reely, Riley and many others as anglicised forms of the one Irish name. He explains the number of spellings by the many dialect pronunciations of the Irish.[1][2]

How do you pronounce O'Reilly?

In English, O'Reilly is pronounced 'oh-RY-lee', with the stress on the middle syllable, and Reilly is 'RY-lee'. In Irish the pronunciation varied by district, which is why Woulfe records spoken forms such as Ó Rághallaigh, Ó Raighilligh and Ó Raoghallaigh.[11][1]

Where does the O'Reilly name come from?

The name comes from County Cavan, the old kingdom of East Breifne, also called Breifne O'Reilly. The O'Reillys first appear around Lough Ramor in 1126. Their seats included Tullymongan above Cavan town and Cloughoughter Castle. Woulfe says that in the 13th and 14th centuries their power spread over all of Cavan and into parts of Meath and Westmeath.[3][1][5]

Is there an O'Reilly coat of arms or clan motto?

No coat of arms belongs to everyone called O'Reilly. Arms belonged to particular families. Wikipedia's East Breifne article associates the motto 'Fortitudine et prudentia' ('with fortitude and prudence') with the O'Reilly rulers' arms, and a defaced O'Reilly crest appears in the arms of County Cavan. Treat any 'O'Reilly family crest' sold for the whole surname with caution.[3][2]

Are the O'Reillys related to the O'Rourkes?

Yes, according to tradition and the genealogies. Woulfe calls the O'Reillys the same stock as the O'Rourkes, and Wikipedia places both in the Uí Briúin Bréifne. The O'Reillys were at first vassals of the O'Rourke kings of Breifne, then rivals. After the division of 1256 the O'Reillys ruled East Breifne (Cavan) and the O'Rourkes West Breifne (Leitrim).[1][6][2][3]

Where in Ireland is the O'Reilly surname most common?

In the 1911 census the largest numbers of O'Reillys lived in Cavan (5,835), Dublin (3,691), Meath (1,993). Relative to population, the name was most concentrated in Cavan (639 per 10,000), Longford (358 per 10,000), Meath (306 per 10,000).

How common was the O'Reilly surname in Ireland?

24,389 people were recorded as Reilly or O'Reilly or Rielly or ORielly in the 1911 census of Ireland, making it the 11th most common surname. That is roughly 0.6% of the population, or 1 person in 180.

Looking for a O'Reilly ancestor?

Search every O'Reilly household in the 1901, 1911 and 1926 censuses, with free images of the original forms.

Search records on irishcensus.com

Evidence

Sources

  1. Patrick Woulfe, Ó Raghailligh, Ó Raghallaigh (1923). Sloinnte Gaedheal is Gall: Irish Names and Surnames
  2. Wikipedia contributors, O'Reilly (2026)
  3. Wikipedia contributors, East Breifne (2026)
  4. Patrick Woulfe, Ó Raghaill (1923). Sloinnte Gaedheal is Gall: Irish Names and Surnames
  5. Wikipedia contributors, Cloughoughter Castle (2026)
  6. Patrick Woulfe, Ó Ruairc (1923). Sloinnte Gaedheal is Gall: Irish Names and Surnames
  7. Wikipedia contributors, Philip O'Reilly (Cavan County MP) (2026)
  8. Wikipedia contributors, Alejandro O'Reilly (2026)
  9. Wikipedia contributors, Andreas O'Reilly von Ballinlough (2026)
  10. Patrick Woulfe, Ó Raoghallaigh (1923). Sloinnte Gaedheal is Gall: Irish Names and Surnames
  11. Wiktionary contributors, O'Reilly (2026)
  12. Census of Ireland 1901 and 1911, and Census of Population 1926, National Archives of Ireland. Counts compiled by surnames.ie from the irishcensus.com transcription (dataset 20260831130351). 1926 data © National Archives of Ireland, CC BY 4.0.
  13. Irish forms cross-checked against Gaois surname data (Fiontar & Scoil na Gaeilge, DCU).

How we compile these figures: methodology.