BIOGRAPHY: The O'Brien and the MacCarthy families had been warring with each other in southwestern Ireland for many years prior to the Norman invasion of Ireland. Iafracoth or Lafeacott was apparently a persuasive woman. When her son-in-law, Maurice fitzGerald, invaded Ireland in 1171, he took by conquest part of the MacCarthy territory. Initially at least, Maurice did not seize any O'Brien territory.
BIOGRAPHY: Most people named O'Brien are descended from Brian macCennéidigh, the Ard-Righ (High King) of Ireland usually known as Brian Bóru. Since Lafeacott O'Brien was born only about 150 years after the army of Brian Bóru defeated the Vikings at Clontarf in A.D. 1014, she was undoubtedly aware of her famous ancestor and careful about the spelling of her name. In later generations, the families of (O) Byrne, O'Broin, and Bryan sometimes changed the spelling of their name to O'Brien.