The O'Brien dynasty was an Irish Clan and noble house of Munster, founded in the 10th century by Brian Boru of the Dál gCais (Dalcassians). After becoming King of Munster, through conquest he established himself as Ard Rí na hÉireann. Brian's descendants thus carried the name Ó Briain, continuing to rule the Kingdom of Munster until the 12th century where their territory had shrunk to the Kingdom of Thomond which they would hold for just under five centuries.