Many Irish surnames that start with O’ come from the Gaelic prefix Ó, which means descendant of. If you’re researching family history, choosing a baby name, or just trying to decode Irish last names, this list gives you 50 real surnames with their Irish forms and the meaning or origin attached to each one.
This guide focuses on established Irish surnames with documented Gaelic roots. In most cases, the original form uses the fada, as in Ó Briain or Ó Néill, even when the English version drops the accent.
If you want the wider surname context first, see our guides to Irish surnames that start with O and what O’ means in Irish surnames. Below, you’ll find a ranked, scannable list of 50 Irish surnames that start with O’, grouped loosely by how often American readers are likely to recognize them.
How To Read Irish Surnames That Start With O’
In Irish, Ó originally marked a descendant line, usually from a notable male ancestor. That does not mean every modern family using O’ descends neatly from one single documented individual. Surnames shifted over centuries, spellings changed under English administration, and some lines dropped or restored the prefix at different times.
You’ll also see variation between the Irish-language form and the Anglicized version. For example, Ó Súilleabháin became O’Sullivan, and Ó Ceallaigh became O’Kelly. If you’re tracing relatives in U.S. census records, passenger lists, or parish registers, checking both forms is often useful. It also helps to compare nearby surname groups, especially if a clerk dropped a prefix or mangled the spelling, so broad lists like Irish last names that start with C and Irish last names that start with M can be surprisingly handy.
50 Irish Surnames That Start With O’
O’Brien Irish form: Ó Briain. This means descendant of Brian and is associated with the dynasty of Brian Boru, the High King of Ireland most closely linked with County Clare and Thomond.
O’Neill Irish form: Ó Néill. It means descendant of Niall, a personal name often linked to the powerful Uí Néill dynasties of Ulster and the north midlands.
O’Sullivan Irish form: Ó Súilleabháin. Usually explained as descendant of Súileabhán, this is one of the best-known Munster surnames, especially in counties Cork and Kerry.
O’Connor Irish form: Ó Conchobhair. The name means descendant of Conchobhar, an old personal name often interpreted as lover of hounds or wolf-lover, depending on the source and element breakdown.
O’Reilly Irish form: Ó Raghallaigh. This surname means descendant of Raghallach and is strongly associated with Breifne, especially County Cavan.
O’Connell Irish form: Ó Conaill. It means descendant of Conall, a personal name often glossed as strong wolf or high valor, depending on the interpretation used.
O’Donnell Irish form: Ó Domhnaill. This means descendant of Domhnall, a name built from elements often translated as world and mighty or rule.
O’Malley Irish form: Ó Máille. The original personal name is usually connected with a chief or noble figure, and the surname is famous in Mayo through the O’Malleys of Clew Bay.
O’Keeffe Irish form: Ó Caoimh. The root personal name relates to gentleness or nobility, though Anglicization makes the path from Irish to English look less obvious than in some other surnames.
O’Callaghan Irish form: Ó Ceallacháin. It means descendant of Ceallachán, a diminutive form built from Ceallach, a personal name with debated older meanings.
O’Keefe Irish form: Ó Caoimh. This is a common spelling variant of O’Keeffe, especially outside Ireland, and it points back to the same Gaelic family line.
O’Hara Irish form: Ó hEaghra. The surname means descendant of Eaghra and is strongly linked with County Sligo.
O’Shea Irish form: Ó Séaghdha. It means descendant of Séaghdha, a personal name usually understood to suggest stateliness or hawk-like qualities, depending on the source.
O’Leary Irish form: Ó Laoghaire. The surname comes from Laoghaire, an old personal name often linked with calf-herder in older Irish naming studies.
O’Flaherty Irish form: Ó Flaithbheartaigh. It means descendant of Flaithbheartach, a name combining elements associated with rulership and bright deeds.
O’Driscoll Irish form: Ó hEidirsceoil. The exact older meaning is not always given consistently, but the surname is a long-established one in west Cork.
O’Mahony Irish form: Ó Mathghamhna. This means descendant of Mathghamhain, a personal name commonly interpreted as bear.
O’Donovan Irish form: Ó Donnabháin. The ancestor name Donnabhán is usually understood as little brown one or dark-haired little one.
O’Farrell Irish form: Ó Fearghail. This means descendant of Fearghal, a personal name commonly glossed as man of valor.
O’Toole Irish form: Ó Tuathail. It means descendant of Tuathal, a personal name often interpreted as ruler of the people or one linked with tribal authority.
O’Boyle Irish form: Ó Baoighill. This is an old Donegal surname, though the underlying personal name is less transparent to modern readers than names like Brian or Niall.
O’Doherty Irish form: Ó Dochartaigh. Usually translated as descendant of Dochartach, with the personal name often connected to obstructive or hard-to-hurt qualities in older scholarship.
O’Rourke Irish form: Ó Ruairc. The surname means descendant of Ruairc, a personal name often related to fame and rulership.
O’Byrne Irish form: Ó Broin. This means descendant of Bran or Broin, a name linked with the raven in Irish tradition.
O’Higgins Irish form: Ó hUiginn. The family name is associated with an older personal name that may have Norse links in some lines.
O’Grady Irish form: Ó Grádaigh. The name means descendant of Grádach, often taken to suggest noble or illustrious qualities.
O’Dwyer Irish form: Ó Dubhuir. This surname is associated with Tipperary, and the original form is usually interpreted through dark and water elements.
O’Halloran Irish form: Ó hAllmhuráin. This means descendant of Allmhurán, a personal name often linked with foreign or seafaring associations.
O’Carroll Irish form: Ó Cearbhaill. It means descendant of Cearbhall, a personal name often interpreted with ideas of hacking or martial vigor in older name studies.
O’Dea Irish form: Ó Deághaidh. The exact early meaning is not always translated in modern surname guides, but it is a recognized Dál gCais surname associated with Clare.
O’Meara Irish form: Ó Meachair. This surname is linked with a personal name often glossed as hospitable or merry, though interpretations vary.
O’Casey Irish form: Ó Cathasaigh. It means descendant of Cathasach, usually understood as vigilant or watchful.
O’Flynn Irish form: Ó Floinn. This means descendant of Flann, an old personal name meaning red or ruddy.
O’Hehir Irish form: Ó hAichir. The exact root meaning is not always preserved in quick-reference surname dictionaries, but the name is old and regionally associated with Clare.
O’Daly Irish form: Ó Dálaigh. This means descendant of Dálach and is especially associated with bardic and learned families in medieval Ireland.
O’Quinn Irish form: Ó Cuinn. The surname means descendant of Conn, a name usually interpreted as chief, sense, or wisdom depending on context.
O’Kane Irish form: Ó Catháin. It means descendant of Cathán, a diminutive name related to battle.
O’Loughlin Irish form: Ó Lochlainn. The ancestor name Lochlann is often linked with Scandinavia or Norse lands in medieval Irish usage.
O’Shaughnessy Irish form: Ó Seachnasaigh. It means descendant of Seachnasach, a personal name often connected with wary or defensive qualities.
O’Friel Irish form: Ó Frighil. This is an Ulster surname, though modern sources do not always preserve a simple plain-English meaning for the original personal name.
O’Gorman Irish form: Ó Gormáin. It means descendant of Gormán, a personal name usually connected with blue or dark coloring.
O’Herlihy Irish form: Ó hIarfhlatha. The root personal name Iarfhlaith is often interpreted as western prince or western ruler.
O’Hanlon Irish form: Ó hAnluain. This surname means descendant of Anluan, a personal name commonly explained with champion or warrior-like associations.
O’Cahill Irish form: Ó Cathail. It means descendant of Cathal, a personal name made from battle elements and widely used in Gaelic Ireland.
O’Madden Irish form: Ó Madáin. This means descendant of Madán, with the original name often treated as a diminutive form in Irish naming traditions.
O’Mulryan Irish form: Ó Maoilriain. It means descendant of Maolriain, with maol in many Irish names referring to a devotee or tonsured follower.
O’Gara Irish form: Ó Gadhra. The ancestor name Gadhra is commonly connected with hound or canine imagery.
O’Clery Irish form: Ó Cléirigh. This surname is associated with hereditary learned families, and the name itself links back to a clerical or scholarly root.
O’Crowley Irish form: Ó Cruadhlaoich. The personal name behind it is often interpreted as hardy warrior.
O’Devaney Irish form: Ó Duibheannaigh. This surname is established in Ulster, though concise translations of the original personal or territorial root are not always consistent across sources.
O’Lunny Irish form: Ó Luinigh. The surname appears in Ulster records and is connected with an older personal name or group designation rather than an easily translated modern word.
Common Patterns In O’ Irish Surnames
A few themes show up again and again. Many names come from an ancestor’s personal name, such as O’Brien from Brian or O’Neill from Niall. Others preserve older descriptive elements, including references to color, battle, animals, rulership, or religious devotion.
You will also notice that some surnames have clear modern translations, while others do not. That is normal. Irish surnames are old, and some root names survive only inside the surname itself, which leaves modern dictionaries with careful interpretations rather than neat one-word definitions. The same thing turns up across other alphabet groups too, including Irish last names that start with R, where old personal names and territorial links still do a lot of the heavy lifting.
The Difference Between O’ and Ó
The apostrophe form O’ is the standard English rendering. The Irish-language form is Ó, with a fada over the O. If you are writing for formal genealogy notes, Irish-language study, or signage in Ireland, using the fada is the accurate form.
In everyday American use, though, families usually write the name without the fada and with the apostrophe. You will also find branches that dropped the prefix entirely after emigration, then restored it later. A family named Sullivan in Boston or Chicago may still connect back to an Ó Súilleabháin line in Kerry or Cork.
Which O’ Surnames Are Most Commonly Recognized In The U.S.?
For most American readers, the easiest names to recognize are O’Brien, O’Connor, O’Neill, O’Sullivan, O’Reilly, O’Donnell, and O’Malley. These show up often in family history searches, Irish American communities, and popular culture. Several also remain common enough in Ireland to appear regularly in modern surname databases and electoral records, so they are not just familiar because of films, school rolls, and pub signs.
Check variant spellings. O’Keeffe and O’Keefe are a good example. A family may appear under more than one English spelling across records.
Look for the Gaelic form. Searching Ó Ceallaigh as well as O’Kelly can help when using Irish-language surname references.
Connect the name to a county. Surnames like O’Driscoll, O’Malley, or O’Doherty often have strong regional associations that narrow the search.
Use official records first. Civil records, parish registers, and census materials are better anchors than family lore that has been polished at reunions for 80 years.
My Irish Ancestry - Five Generations - Genealogy Workbook.: A beautifully designed Genealogy Workbook. The perfect start for your journey back in time to discover your family tree.
Final Thoughts On 50 Irish Surnames That Start With O’
The big thing to remember is simple: O’ surnames are usually Anglicized forms of Irish surnames beginning with Ó, meaning descendant of. Some are tied to famous dynasties, some to local ruling families, and some preserve personal names whose meanings are partly lost in the fog of early medieval language. Very Irish, in other words.
If a surname on this list matches your own, the next step is not guessing. It is checking the Irish form, looking for spelling variants, and pairing the name with real records and a likely county base. That is where the story usually gets interesting.
Brought to You by Ireland Wide
At Ireland Wide, our aim is to bring authentically Irish insights to you, wherever you are.
Popular Posts
Here are some of our most popular posts about Irish culture, heritage, and travel.
