O'Reilly & Reilly Irish Surname History: Ó Raghallaigh Origins

O'Reilly and Reilly: the short answer

O'Reilly and Reilly are documented anglicized forms of Irish Ó Raghallaigh, meaning “descendant of Raghallach.” Patrick Woulfe records both spellings, along with historical Riley forms, and the Ó Raghallaigh dynasty ruled in Breifne and later East Breifne, the territory of modern County Cavan.

For gifts in either spelling, browse the O'Reilly & Reilly Irish family heritage gifts collection.

What does Ó Raghallaigh mean?

Woulfe explains Ó Raghallaigh as “descendant of Raghallach.” Raghallach was the personal name of the family's ancestor, and Woulfe gives no further gloss for it. He places the O'Reillys of the same stock as the O'Rourkes, their neighbors and frequent rivals in Breifne.

Breifne, East Breifne and County Cavan

The strongest historical association is with the Ó Raghallaigh dynasty of Breifne. The Dictionary of Irish Biography records Cathal Ó Ragallaig (d. 1256) as king of Breifne and shows the family as an important regional power. In later medieval history the O'Reilly rulers held East Breifne, centered on modern County Cavan. Woulfe records that in the thirteenth and fourteenth centuries they extended their rule over the whole of Cavan and parts of Meath and Westmeath, kept their independence until the reign of James I, and held considerable property until the Cromwellian confiscations. Five O'Reillys served as archbishops of Armagh.

For family research, County Cavan is the heartland, with neighboring parts of Meath and Westmeath close behind. Parish and townland evidence places a family within that landscape.

O'Reilly, Reilly and Riley spellings

Woulfe records several anglicized forms of Ó Raghallaigh, including O'Reilly, Reilly and Riley. Prefixes and spellings could change or be recorded inconsistently over time. In genealogy work, search the spelling used by a family in each generation and compare it with place, relatives and dates.

How to research an O'Reilly or Reilly family

  1. Start with a known person and place. Work backward from civil certificates, parish records, census returns and family documents.
  2. Search multiple spellings. Check O'Reilly and Reilly, and add historical Riley/Reily forms as the record trail reaches back.
  3. Use place evidence. If the family points to County Cavan, compare townlands, parishes and nearby households in the 1901 and 1911 census and other Irish record sets.
  4. Follow migration records. Passenger lists, naturalization files, vital records and obituaries abroad can help connect an emigrant family back to a specific Irish locality.
  5. Record family documents. Preserve photographs, certificates and oral history alongside the civil and church records that establish each generation.

Tony O'Reilly in Irish sport

Sir Anthony “Tony” O'Reilly (1936–2024) brought the surname into twentieth-century Irish sporting and business history. A wing for Ireland and the British & Irish Lions, he later became chairman and chief executive of H.J. Heinz. The Irish Rugby Football Union's tribute records his international career and wider contribution.

Migration and the wider diaspora

O'Reilly and Reilly families appear far beyond Ireland, including in Britain, North America and Australia. John Boyle O'Reilly (1844–1890), born at Dowth in County Meath, escaped penal transportation in Western Australia in 1869 and became a celebrated poet and editor of the Boston Pilot. Spellings often shifted in migration, so the strongest research path is to identify the emigrant generation, document its destination records, and then connect it back to a specific Irish place.

Frequently asked questions

Are O'Reilly and Reilly the same surname?

Yes. Both are anglicized forms of Ó Raghallaigh. Search both spellings when following an Irish family through older records.

Is Riley connected to O'Reilly?

Yes. Woulfe records Riley among the historical anglicized forms of Ó Raghallaigh. Riley is also an English surname taken from local place-names, so an English-born Riley line can have its own separate origin.

Where is the surname most strongly associated in Ireland?

County Cavan. The O'Reillys ruled Breifne and then East Breifne, and Cavan remains the county most closely tied to the name.

Sources and records