O' Prefix

The prefix Ó (anglicized as O') is the modern Irish form of the Old Irish word ua, meaning 'grandson' or, by extension, 'descendant of'. It indicates that the surname bearer claims descent from the named ancestor, who was typically a prominent chieftain or respected figure. O'Brien, for example, derives from Ó Briain, meaning 'descendant of Brian' — a reference to the legendary High King of Ireland, Brian Boru. The Ó prefix is somewhat more common than Mac in modern Irish surnames, especially in Munster and Connacht.

Historical Evolution

The ua form appears in historical records from at least the ninth century. By the twelfth century, it was being used to form stable hereditary surnames for ruling dynasties. During the period of Anglicization under British rule, the Ó prefix was frequently dropped entirely from surnames — O'Sullivan became Sullivan, O'Neill became Neill — or was simplified to a plain O without the apostrophe or the fada (accent). The late nineteenth and early twentieth century saw a strong revival movement to restore the Ó prefix, coinciding with the broader Gaelic Revival and the Irish independence movement.

Grammatical Variations

Like the Mac prefix, Ó changes form depending on gender: a man is Ó Briain, but his sister or daughter is Ní Bhriain (Ní being a contraction of Iníon Uí, 'daughter of the descendant of'), and a married woman takes Bean Uí Bhriain. These grammatical mutations (lenition) are a feature of the Irish language's initial consonant mutation system. In everyday anglicized usage the apostrophe form O' predominates, though the Irish state recognizes and encourages the Ó form in official documents.