Kinds of surname

O' (Ó, Ua)

Also written or known as O', Ó and Ua

An Irish Gaelic prefix meaning "grandson" or "descendant of".

Irish surnames with O' come from Ó (older Ua), meaning grandson or descendant, so O'Brien means "descendant of Brian", here Brian Boru. Many families dropped the O' under English rule and restored it from the late 19th century.