The Battle of Glen Mama.

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999
 

In this battle Brian Boroimhe (Brian Boru) established his power as High King of Ireland. In 998 Brian and Mael Seachnaill II were the two principal chieftains in Ireland, they agreed a settlement which gave Brian the south and Mael the north. Mael put aside his wife, Gormfhlaith, widow of the Viking leader Olaf, Gormfhlaith sought help from from her brother Mael Mordha, urging him to ally himself with her Viking son Sitric, lord of Dublin against Brian and Mael Seachnaill.

The rebellion was utterly crushed at Glen Mama, where Brian's force's routed the combined armies of Mael Mordha and Sitric. Brian used diplomacy to ease the tension, he married Gormfhlaith, reinstated Sitric in Dublin and made Mael Mordha king of Leinster.

His diplomatic efforts achieved a pease of sorts for fifteen years, in 1014 Brian once again faced the Vikings and their allies at the Battle of Clontarf (Dublin) where he defeated them although losing his own life, slain by a group of Vikings fleeing the battle.