Reilly — Ó Raghallaigh | Meaning, Origin & Irish Heritage

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The surname Reilly is the anglicised form of the Irish Ó Raghallaigh — meaning “descendant of Raghallach”. Derived from the Irish personal name Raghallach, whose etymology is debated — one interpretation is ‘sociable’ or ‘gregarious’, but the name may be pre-Celtic and of uncertain origin.^[1]

Meaning & Etymology

Derived from the Irish personal name Raghallach, whose etymology is debated — one interpretation is ‘sociable’ or ‘gregarious’, but the name may be pre-Celtic and of uncertain origin.^[1]

The Ó Raghallaigh family held the kingdom of Breifne (modern County Cavan and parts of Leitrim) for nearly 500 years.^[2]

Reilly and O’Reilly are interchangeable — the same surname with and without the prefix.

Historical Reilly Septs

There is rarely a single family behind a major Irish surname. Edward MacLysaght — the gold-standard source for Irish surname history — identifies distinct historical septs that all anglicised to Reilly, often with no kinship to one another.^[2]

The Kingdom of East Breifne

The Ó Raghallaigh ruled the kingdom of Breifne Uí Raghallaigh — roughly modern County Cavan — from the early 12th century until the Flight of the Earls in 1607. Their capital was at Tullymongan Hill near modern Cavan town. They survived as a Gaelic royal family longer than most, finally losing their lands in the Plantation of Ulster.^[1][2]

The Reilly coinage

In the late medieval period the O’Reillys minted their own coinage — known as ‘Reilly’s money’ — in defiance of English royal authority. The phrase ‘the life of Riley’ in English slang is sometimes (debatably) traced to this period of O’Reilly prosperity.^[1][2]

Famous Bearers

• Myles ‘the Slasher’ O’Reilly (d. 1644) — Cavan chief killed defending Finea Bridge in the Confederate Wars; a legendary Irish folk hero.
• John Boyle O’Reilly (1844-1890) — Meath-born Fenian, transported to Australia, escaped to America, became a celebrated poet and newspaper editor in Boston.
• Saoirse Ronan (maternal side) — the Oscar-nominated actress’s mother is a Reilly from Carlow.
• Bill O’Reilly — American broadcaster and author; Irish-American descendant.
• Tony Reilly (Sir Anthony O’Reilly) — Dublin-born rugby international and business tycoon, former CEO of H.J. Heinz.

Spelling Variants & Anglicisations

Over centuries of anglicisation, translation, and emigration, the Reilly name has taken many forms in English and Irish:

• Reilly · O’Reilly · OReilly
• Riley (common American anglicisation)
• Ó Raghallaigh · Ó Raghailligh (historical and modern Irish)

Where to Visit if Your Name is Reilly

If you carry the Reilly name and want to walk the ground your ancestors once held, here are the regions of Ireland that are your strongest historical anchors:

• County Cavan — the ancestral heartland of Breifne. Visit Tullymongan Hill (the ancient inauguration site), Lough Oughter with its island castle ruins, and the Cavan Burren Park.
• County Leitrim — the western half of historical Breifne. Walk Glencar Waterfall (beloved of W.B. Yeats) and visit Lough Allen and the source of the Shannon.
• Finea, County Westmeath — the site of the 1644 battle where Myles ‘the Slasher’ O’Reilly was killed. A roadside monument marks the spot.

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Sources

1. Wikipedia: Wikipedia (en) — consolidated etymology and modern rank [link]
2. Maclysaght: MacLysaght, Edward. Irish Families: Their Names, Arms and Origins. 1957, Hodges Figgis. The standard scholarly reference on the origin and distribution of Irish surnames.

All facts above are sourced from the named references listed above. Page last verified April 2026.

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