O'Brien is a common surname of Irish origin.
The name is Gaelic, and its original language version is Ua Briain or Ó Briain (pronounced: Ew-Breen/Oh-Breen), meaning 'descendant of Brian'. It carries the sense ‘eminence’, i.e. ‘exalted one’. The Brian referred to is in most cases the ancient Irish king Brian Boru, or in Gaeilge - Brian Bórumha mac Cennétig.
This O'Brien family ruled over Munster after Brian Boru's death, and later supplied the rulers of Thomond until the area became incorporated into the English holdings in Ireland in the Tudor period (but see the de Clare family).