Muircheartach Ua Briain (old spelling: Muirchertach Ua Briain, also known as Murtough O'Brien), son of Toirdelbach Ua Briain and great-grandson of Brian Bóruma, was King of Munster and later self-declared High King of Ireland.
Between 1086 and 1101, Muirchertach consolidated and strengthened his position as King of Munster. He went on forays into Mide and Leinster in 1089 and took the kingship of Leinster and fought for the Viking Kingdom of Dublin. In 1093, he accepted the submission of Domnall mac Flainn Ua Maíl Shechnaill, the Uí Néill king of Tara, and also made peace with his brother Diarmait at Cashel.
In 1094, Muirchertach fought the kings of Leth Cuinn and Gofraid Crobán, King of Dublin and the Isles. He went with his army to Dublin and banished Gofraid, and brought about the killing of Domnall Ua Maíl Shechnaill. He asserted supremacy over the Uí Néill kingdom of Mide.
In 1101 he declared himself High King and traveled the island provinces. It was in this year that he gave the fortress at on the rock of Cashel as a gift to the Church.
In 1114 the king became sick to the point where "he became a living skeleton". In response to the king's misfortune, his brother Diarmait took control of the kingship of Munster and banished Muirchertach. The following year Muirchertach regained his strength and undertook a campaign to regain control of Munster and successfully captured Diarmait. Only later did the king regain control of Munster.
In 1119, Muirchertach Ua Briain died.