The
O'Brien dynasty (Classical Irish , (Modern Irish
Ó Briain, IPA: /oːˈbʲɾʲiənʲ/), genitive
Uí Bhriain, IPA: /iːˈβʲɾʲiənʲ/) are a
royal and
noble house founded in the 10th century by
Brian Boru of the
Dál gCais or Dalcassians. After becoming
King of Munster, through conquest he established himself as
Ard Rí na hÉireann (
High King of Ireland). 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.