Brian Boru

Brian Boru was an Irish king who ended the domination of the High Kingship of Ireland by the U Nill. Building on the achievements of his father, Cenntig mac Lorcain, and especially his elder brother, Mathgamain, Brian first made himself King of Munster, then subjugated Leinster, eventually becoming King of Ireland. He is the founder of the OBrien dynasty.

Many Irish annals state that Brian was in his 88th year when he fell in the Battle of Clontarf in 1014. If true, this would mean that he was born as early as 926 or 927. Other birth dates given in retrospect are 923 or 942.

Source: Wikipedia