The O'Brien and the MacCarthy families had been warring with each otherin southwestern Ireland for many years prior to the Norman invasion ofIreland. Iafracoth or Lafeacott was apparently a persuasive woman. Whenher son-in-law, Maurice fitzGerald, invaded Ireland in 1171, he took byconquest part of the MacCarthy territory. Initially at least, Maurice didnot seize any O'Brien territory. Most people named O'Brien are descended from Brian mac Cennéidigh, theArd-Righ (High King) of Ireland usually known as Brian Bóru. SinceLafeacott O'Brien was born only about 150 years after the army of BrianBóru defeated the Vikings at Clontarf in A.D. 1014, she was undoubtedlyaware of her famous ancestor and careful about the spelling of her name.In later generations, the families of (O) Byrne, O'Broin, and Bryansometimes changed the spelling of their name to O'Brien.