Brian was born c. 940 in north Munster, the youngest son of Cennétig, king of the Dál Cais. In 976, he succeeded his murdered brother Mahon or Mathgamain, and claimed the kingship of Munster. Since 800, the Vikings or Norsemen had founded ports around Ireland, and their longboats easily penetrated Munster. Brian was the first Irish ruler to assemble a fleet to defend and extend his territory. From his seat at Kincora (now Killaloe, Co Clare), near Lough Derg, he was able to sail up the Shannon to ravage Connacht, Meath and Breifne.
Brian's main Irish rival was Malachy, king of Meath, who had become high king at Tara in 980. Malachy had driven the Viking Olaf out of Dublin; Olaf's son, Sitric, eventually ruled Dublin under Malachy. In 997, the two kings agreed to divide Ireland between them, Brian gaining suzerainty over Dublin. In 999, the Leinstermen joined with Sitric against Brian, only to suffer defeat at Glenmama, Co Wicklow.
Brian married Gormflath (who had been married successively to Olaf and Malachy), gave his own daughter to Sitric, and claimed the northern half of Ireland. Malachy failed to marshal the support of the Northern Uí Néill, and in 1002 conceded the high kingship. Collecting tribute and hostages on his royal journeys, Brian acquired the name Boru (Brian of the Tributes). In 1005, he gave twenty ounces of gold to the Church, and recognised the archbishop of Armagh as primate of all Ireland.
In 1013, the unfaithful Gormflath roused Sitric and her brother Maelmora, king of Leinster, against Malachy. Brian came to Malachy's aid, but failed to take Dublin and retired to Kincora. Sitric made an alliance with Sigurd, earl of Orkney, who delivered 2,000 Norse soldiers and was promised Gormflath in marriage. The Leinstermen and Vikings were routed at Clontarf, outside Dublin, on 23 April 1014, but the aged Brian was slain in his tent. His death foreshadowed new dynastic conflicts which left Ireland vulnerable to the Norman invasion in 1169.

See:
A tablet in the outer wall of the north transept of Armagh Cathedral marks Brian's reputed grave.

From the Appletree Press title: Famous Irish Lives.
Also from Appletree: Irish Museums and Heritage Centres.