# O'Reilly

**O'Reilly** is an Irish surname, the anglicized form of the Gaelic **Ó Raghallaigh**, meaning "descendant of Raghallach." It was the name of the ruling family of East Breifne, the territory of modern [County Cavan](https://www.edgepedia.org/county-cavan), and it remains one of the most common surnames in Ireland.

| Key fact | Detail |
|---|---|
| Gaelic form | Ó Raghallaigh, "descendant of Raghallach"; the Ó prefix means "descendant of"<sup>[1](https://forebears.io/surnames/o%60reilly)</sup> |
| Historical territory | East Breifne (Breifne Uí Raghallaigh), centered on County Cavan; also strong presence in Meath and Westmeath<sup>[2](https://celticancestrygifts.com/blogs/news/oreilly-irish-surname-history-origins)</sup> |
| Modern frequency | Reilly is the 11th most popular surname in Ireland, with about 27,000 namesakes; the spelling O'Reilly ranks 20th there, with 19,198 bearers<sup>[1](https://forebears.io/surnames/o%60reilly)</sup> |
| 1911 census | 4,588 people named O'Reilly in Ireland, about 0.105% of the population, 172nd most common surname overall<sup>[3](https://irishhistory.com/surnames/oreilly/)</sup> |
| Distinctive right | The East Breifne lords minted their own coinage; a 1457 act by the Lord Deputy prohibited circulation of "O'Reilly's money" in the English Pale<sup>[4](https://www.libraryireland.com/irish-families/oreilly.php)</sup> |
| Common variants | O'Reilly, Reilly, Riley, Reily, Reeley<sup>[5](https://synpromedia.com/surnames/irish/reilly)</sup> |

## Meaning and etymology

The surname comes from the Irish Ó Raghallaigh, in which Ó marks descent and Raghallach (Old Irish Roghallach) was a personal name. Dictionaries differ on what that name meant. Edward MacLysaght, former Chief Herald of Ireland and author of *A Guide to Irish Names* (1964), glossed it as "the valiant or prosperous one"<sup>[1](https://forebears.io/surnames/o%60reilly)</sup>; Elsdon Coles Smith's *Dictionary of American Family Names* (1956) gave "the sportive one"<sup>[6](https://forebears.io/surnames/oreilly)</sup>. The *Dictionary of American Family Names* edition by Patrick Hanks treats the etymology as unexplained<sup>[7](https://www.familysearch.org/en/surname?surname=o%27reilly)</sup>, and one Irish heritage account notes the name may be pre-Celtic and of uncertain origin<sup>[8](https://lovetovisitireland.com/irish-names/reilly/)</sup>. A popular etymology separates the name into *rath* (prosperity, grace) and *gal* (valor), giving "valiant in prosperity," but the older dictionaries do not support this analysis<sup>[5](https://synpromedia.com/surnames/irish/reilly)</sup>.

The Ó prefix is the Irish equivalent of the Scottish and Irish "Mac-" ("son of"), the Anglo-Norman "Fitz-", and the Welsh "Ap-"<sup>[1](https://forebears.io/surnames/o%60reilly)</sup>. In standard [Irish orthography](https://www.edgepedia.org/irish-orthography) the base form is Ó Raghallaigh, with inflected forms such as Uí Raghallaigh and Ní Raghallaigh; the Gaois linguistic database of Irish-language surnames treats Ó Raghallaigh, O'Reilly, and Reilly as one surname cluster<sup>[9](https://ans-names.pitt.edu/ans/article/download/2251/2248)</sup>.

## The dynasty of East Breifne

The O'Reillys were the chieftain family of County Cavan, related to the O'Rourkes, their neighbors and rivals in Breifne<sup>[7](https://www.familysearch.org/en/surname?surname=o%27reilly)</sup>. The family claimed descent from Brian, a Milesian king of Connaught, and traced the surname to an ancestor called Ragallach who lived at the beginning of the eleventh century; his grandson Targaille, prince of East Brefny, was reportedly the first to use the name Ua Ragallagh<sup>[10](https://forebears.io/surnames/o%60reilly)</sup>.

The annalistic record confirms the family's prominence from the twelfth century. Early bearers include Goffraigh hUa Raghallaigh, lord of Breifni, in 1161, and Cathal O Raighlidh in 1226, both in the Annals of Ulster; later bearers include Mael-Sechlainn O Raighligh, king of Muinter-Maelmordha, in 1328, and Fergal mac Tomais moir hUi Raghailigh, king of Breifne, in 1450<sup>[11](https://downloads.freemdict.com/%E5%B0%9A%E6%9C%AA%E6%95%B4%E7%90%86/%E5%85%B1%E4%BA%AB2020.5.11/content/1_english/3_Oxford%20series/The%20Oxford%20Dictionary%20of%20Family%20Names%20in%20Britain%20and%20Ireland/The%20Oxford%20Dictionary%20of%20Family%20Names%20in%20Britain%20and%20Ireland.pdf)</sup>. Cú Chonnacht O'Reilly (Ó Raghallaigh), who died in 1367, was king of East Breifne, succeeding his brother Risderd on the latter's death in 1349<sup>[12](https://www.dib.ie/node/22812)</sup>.

Under the O'Reillys, Cavan acquired the Gaelic designation Tír na Raghallaigh, "the land of O'Reilly"<sup>[2](https://celticancestrygifts.com/blogs/news/oreilly-irish-surname-history-origins)</sup>. Their power was sufficient for the family to mint its own coinage; in 1457 the Lord Deputy, Thomas Fitzgerald, Earl of Kildare, passed acts prohibiting the circulation of the Irish coinage called "O'Reilly's money" in the English Pale<sup>[4](https://www.libraryireland.com/irish-families/oreilly.php)</sup>. The same source records that Maolmordha (Mulmora), Latinized "Milesius" and Anglicized "Miles" or "Myles," was a favorite [Christian name](https://www.edgepedia.org/christian-name) among the O'Reillys<sup>[4](https://www.libraryireland.com/irish-families/oreilly.php)</sup>.

## Distribution

The surname remains most concentrated in its historical heartland. Forebears data place Ireland first for the O'Reilly spelling, with 19,198 bearers at a frequency of 1 in 245 and a rank of 20 in the country, followed by England (9,869), Australia (5,882), the United States (5,144), and Canada (4,354)<sup>[1](https://forebears.io/surnames/o%60reilly)</sup>. Heritage accounts describe the clan as concentrated in the Midlands of Ireland, particularly County Cavan and County Westmeath, with diaspora communities in the United States, Canada, and Australia<sup>[13](https://www.thenamedb.com/surname/o-reilly)</sup>.

The anglicized spelling has a history of its own in Britain: the earliest record in English sources is a reference to one William Orailly in the Calendar of Patent Rolls in 1451, though in Ireland the name is certainly older<sup>[1](https://forebears.io/surnames/o%60reilly)</sup>.

## Variants and unrelated names

O'Reilly, Reilly, and Riley are documented anglicized forms of Ó Raghallaigh<sup>[14](https://celticancestrygifts.com/blogs/news/reilly-irish-surname-history-origins)</sup>, and a long list of spelling variants has been recorded, including O'Riley, Reighly, Reiley, Reilley, Reily, Ryley, and others<sup>[15](https://occgs.com/projects/rescue/family_files/files/RILEY%20Family.pdf)</sup>.

Not every Riley is Irish. The anglicized forms Riley and Ryley are also borrowed from a native English surname based on a common place-name type (in Devon, Lancashire, and elsewhere) meaning "clearing where rye grows," so an English-born Riley line has a separate origin<sup>[1](https://forebears.io/surnames/o%60reilly)</sup>. The Irish O'Reilly has also been confused in anglicized form with Ó Raithile of Munster, usually anglicized as O'Rahilly, which is a different surname cluster<sup>[7](https://www.familysearch.org/en/surname?surname=o%27reilly)</sup>.

## References

1. [O'Reilly Surname Origin, Meaning & Last Name History, Forebears](https://forebears.io/surnames/o%60reilly)
2. [O'Reilly Irish Surname: History, Origins & Heritage, Celtic Ancestry Gifts](https://celticancestrygifts.com/blogs/news/oreilly-irish-surname-history-origins)
3. [O'Reilly family name history and map, IrishHistory.com](https://irishhistory.com/surnames/oreilly/)
4. [O'Reilly Family, A Genealogical History of Irish Families, Library Ireland](https://www.libraryireland.com/irish-families/oreilly.php)
5. [O'Reilly / Reilly — Irish Surname Origin, County Roots & Family History, SynproMedia](https://synpromedia.com/surnames/irish/reilly)
6. [Oreilly Surname Origin, Meaning & Last Name History, Forebears](https://forebears.io/surnames/oreilly)
7. [O'reilly Name Meaning and Family History, FamilySearch](https://www.familysearch.org/en/surname?surname=o%27reilly)
8. [Reilly — Ó Raghallaigh, Meaning, Origin & Irish Heritage, Love to Visit Ireland](https://lovetovisitireland.com/irish-names/reilly/)
9. [Developing the Gaois Linguistic Database of Irish-language Surnames, Names: A Journal of Onomastics](https://ans-names.pitt.edu/ans/article/download/2251/2248)
10. [O'Reilly Surname - Origin & History, TheNameDB](https://www.thenamedb.com/surname/o-reilly)
11. [The Oxford Dictionary of Family Names in Britain and Ireland](https://downloads.freemdict.com/%E5%B0%9A%E6%9C%AA%E6%95%B4%E7%90%86/%E5%85%B1%E4%BA%AB2020.5.11/content/1_english/3_Oxford%20series/The%20Oxford%20Dictionary%20of%20Family%20Names%20in%20Britain%20and%20Ireland/The%20Oxford%20Dictionary%20of%20Family%20Names%20in%20Britain%20and%20Ireland.pdf)
12. [O'Reilly (Ó Raghallaigh), Cú Chonnacht, Dictionary of Irish Biography](https://www.dib.ie/node/22812)
13. [O'Reilly Surname - Origin & History, TheNameDB](https://www.thenamedb.com/surname/o-reilly)
14. [O'Reilly & Reilly Surname History, Celtic Ancestry Gifts](https://celticancestrygifts.com/blogs/news/reilly-irish-surname-history-origins)
15. [The Riley Newsletter, Oklahoma County Genealogical Society](https://occgs.com/projects/rescue/family_files/files/RILEY%20Family.pdf)

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*Topic: Encyclopedia › Society and history › Languages and linguistics › Names and onomastics › Individual surnames (by language and origin)*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 25, 2026 · Edited: Sep 30, 2026; Oct 15, 2026; Oct 25, 2026 · Last review: Oct 25, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license