Legendary High King of Ireland who defeated the Vikings at the battle of Clontarf.
Disclaimer: Much of what has been writen about Boru is embellished in the manner of King Arthur.
Brian Boru (Briain Boroimhe in Gaelic) was born in 940AD to Kennedy of Thormond in what is now county Clare. He was a member of the Dál gCais (Dalcassian) tribe who were periodically raided by Vikings. At an early age Brian witnessed Vikings murder his mother. He advocated war against the Norse but his brother Mahon preferred peaceful engagement.
At that time, Ireland was a patchwork of clans. The most powerful was the Uí Neill (O' Neill) of Ulster. Vikings had established settlements in coastal areas (e.g. Dublin, Limerick). Many Gaelic chieftains paid tribute to these Norsemen. Ireland lacked a unifying force that would drive them out.
As soon as he was old enough, Brian battled against the Vikings and forced the Dane Ivar (also known as Imar) out of Limerick. However, Ivar returned and killed Boru's brother. Boru killed Ivar in a duel and established his authority over the southern province of Munster.
More chieftains rallied to Boru's cause. In 1002 he was crowned High King of Ireland in Cashel (a powerful castle built on a rock in the middle of a plain and still impressive today). He set about restoring monasteries and churches that had been destroyed by the Norse invaders. By repute, Brian enjoyed four wives and thirty concubines (greatly complicating the succession). The courtly harp which he favoured is a national symbol of Ireland today.
However, Boru had many enemies and Maelmordha of Leinster rebelled and allied with the Dublin Vikings. Vikings from as far afield as Normandy, Iceland and the Orkneys rallied to Maelmordha's cause. The Gaelic chieftains of Munster, Connaught and even Scotland rallied under Boru's banner.
Battle was joined at Clontarf (today it is within Dublin city) on Good Friday, 1014. The Irish won a decisive victory killing about seven thousand Vikings and forcing them to flee in their longboats. Boru's son Murchadh was killed as he led the elite Dalcassian knights in combat. Tragically for Boru, a party of retreating Danes stumbled upon Boru's tent (where he had sat out the battle due to his advanced age) and murdered him (but of course not before he had dispatched three Danes with his mighty two-handed sword).
The defeated Vikings never reestablished their power in Ireland again. However, Boru's death had left several heirs vying for the High Kingship. Subsequent High Kings were never as powerful as Boru; a situation that made Norman conquest all the easier when they landed a century and a half later.