Toirdhealbhach Ua Briain (old spelling: Toirdelbach Ua Briain), anglicised Turlough O'Brien, was King of Munster and effectively High King of Ireland. A grandson of Brian Bóruma, Toirdelbach was the son of Tadc mac Briain who was killed in 1023 by his half-brother Donnchad mac Briain.
For the first forty years of his life nothing is known of Toirdelbach. It was not until the 1050s that he found allies in Connacht and in Leinster, particularly the powerful King of Leinster Diarmait mac Maíl na mBó, who would aid his claims to be ruler of Munster. It took perhaps ten years of sustained attack to remove his uncle Donnchad from power, and send him into exile, and to place Toirdelbach in power in Munster as Diarmait's faithful ally.
On Diarmait's death Toirdelbach took over the reins of power, establishing himself as ruler of more than half of Ireland. While not a great military leader, he was a capable politician whose influence extended as far north as Ulaid and who made and unmade Kings of Connacht. He died after more than two decades in power, following a lengthy illness, still in control of events. His son Muirchertach Ua Briain would be the leading king of his day, and his grandson Toirdelbach Ua Conchobair would be greater yet.