O'Reilly (Irish: Ó Raghallaigh)[1] is a group of families, ultimately all of Irish Gaelic origin, who were historically the kings of East Bréifne in what is today County Cavan. The clan were part of the Connachta's Uí Briúin Bréifne kindred and were closely related to the Ó Ruairc (O'Rourkes) of West Bréifne. O'Reilly is ranked tenth in the top twenty list of most commom Irish surnames.

Ó Raghallaigh
Parent houseUí Briúin Bréifne
CountryKingdom of Connacht
FounderRagheallach mac Cathalan
Current headMyles Hugh O'Reilly
Final rulerMaelmordha O'Reilly
  • King of East Bréifne
  • King of Muintir Maelmórdha
  • Conde de O'Reilly
Cadet branchesBrady

It is also the patronymic form of the Irish name Reilly (Irish Gaelic: Uí Raghaile).[2] It is commonly found throughout Ireland, with the greatest concentration of the surname found in County Cavan followed by Longford, Meath, Westmeath, Fermanagh and Monaghan, and the Province of Leinster.

Naming conventions

Male Daughter Wife (Long) Wife (Short)
Ó Raghallaigh[3] Ní Raghallaigh Bean Uí Raghallaigh Uí Raghallaigh


It is usually anglicised as Reilly, Riley and O'Reilly. The original form of the name, Ó Raghallaigh, denotes "from/of Raghallach", the name Raghallach thought to be derived from the compounds ragh (meaning "race") and ceallach (meaning "sociable").

The Ó Raghallaigh family were part of the Connachta, with the eponymous Raghallach said to have died at the Battle of Clontarf in 1014. The family became the kings of East Breifne, modern-day County Cavan and County Longford.

The name is common and widespread throughout Ireland, ranked 11th most common in 1890[4] and in 1997.[5]

The O'Reilly Clan Chieftain to this day is at odds with the O'Rourke Clan Chieftain because both clans viciously contest each other for the title Prince of Breifne. This was settled in 1994 when the Chief Herald of Ireland made the O'Rourke Chief the Prince of Breifne, but the Office of the Chief Herald stopped granting courtesy titles to Gaelic Chiefs in 2003 (see O'Rourke). In 2017, with the election of the new O'Reilly Chief, the rivalry has been rekindled.



People with the surname O'Reilly include:[6]


  • Christopher O'Riley (born 1956), American classical pianist and public radio show host
  • Don O'Riley (1945–1997), Major League Baseball pitcher
  • Bunny Wailer (born 1947), reggae musician sometimes known as Bunny O'Riley
  • Jon Riley (1824–1879), US army deserter also known as John O'Riley


Fictional people

  • Mr. O'Reilly, Fictitious Irish character/builder portrayed by the Irish actor David Kelly on the episode "The Builders" from Fawlty Towers
  • Aloysius Umbongo N'Danga O'Reilly, fictitious character in the song "Baguette Dilemma for the Booker Prize Guy" by the band Half Man Half Biscuit on their 2014 album Urge for Offal
  • Corporal Walter "Radar" O'Reilly, character in the M*A*S*H media franchise

Other entities

See also


  1. Information on the last name O'Reilly
  2. Information on the last name O'Reilly (2)
  3. "Ó Raghallaigh". Sloinne. 5 December 2015.
  4. Matheson, Robert E. (1894), Special report on surnames in Ireland, with notes as to numerical strength, derivation, ethnology, and distribution; based on information extracted from the indexes of the General register office. Appendix to the Twenty-Ninth Detailed Annual Report of the Registrar-General of Marriages, Births, and Deaths in Ireland, Dublin: For Her Majesty's Stationary Office by Alexander Thom & Co. (Limited), p. 9 (accessible online)
  5. Murphy, Sean J. (2014), A Survey of Irish Surnames 1992–97. In: Studies in Irish Genealogy and Heraldry, Windgates, County Wicklow, pp. 14–29 (first published 2008, last revision 2014), here p. 27 (accessible online)
  6. "O'Reilly coat of arms, family crest and O'Reilly family history". irishsurnames.com. Retrieved 1 January 2008.
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