He was King of Thomond from 1343, after his second cousin, Diarmaid, was deposed, until 1350.

In a contest for the crown of Thomond between two members of the O'Brien family, Murtogh O'Brien and Brian Bawn O'Brien, Murtogh finally defeated Brian Bawn, who was supported by the O'Carroll of Ely. This battle took place at Dysert O'Dea in Co Clare 10 May 1318. After this battle, Brian Bawn and his friend, Mahon of Inchiquin, retired to East Thomond, where they acquired territory in Ara Thire, or O'Brien Ara, which was held by their posterity during succeeding centuries. This Brian Bawn afterwards became King of Thomond. The O'Briens of Ara in earlier times had their chief stronghold in Pallas, but in later times they possessed a celebrated manor-house in Castletown. They are said to have possessed eight castles in Ara, including Pallas Castle, 14th century, Castletown 16th century, Ballina, Kilcolman, Knockaneinfhinn or Birdhill, and Castlelough.27