Brian Boru or Brian Boroimhe (both: brī´ən, brēn; bərōō´, bərō´), 940?–1014, king of Ireland. A clan prince, he succeeded his brother Mathghamhain, who had seized the throne of Munster from the Eogharacht rulers (963). Brian subjugated all Munster, then extended his power over all S Ireland, and in 1002 became high king of Ireland by right of conquest. As his power increased, relations with the Norse rulers on the Irish coast grew steadily worse. Sitric, king of the Dublin Norse, formed against Brian a coalition of the Norse of Ireland, the Hebrides, the Orkneys, and Iceland as well as Brian's Irish enemies. On Good Friday (Apr. 23), 1014, Brian's forces met and annihilated the allies at Clontarf, near Dublin. Soon afterward he was murdered in his tent. Brian's victory broke the Norse power in Ireland forever, but Ireland fell into anarchy.