Brian Boru was born in what is now County Clare, Ireland, c 940 AD, and was the twelfth son of Kennedy of Thomond. At this time in Irish history the country was overrun by Danish Vikings, who had already captured Dublin and had begun to settle in various parts of Ireland.
Boru was the chief of a fairly insignificant tribe known as the Dalcassian. His eldest brother Malachi (or Mahon) succeeded to the throne of Munster. When the Danes swept across Ireland, Brian united with his brother to drive them out. When Malachi was killed by treachery, Brian became the undisputed King of Munster.
After establishing dominance in his native Munster, Boru gradually gained influence over a large part of Ireland, using a combination of diplomacy and warfare, and was considered by many to be the overlord of all Ireland. In 1014, when he was in his seventies, Boru led the Irish in a final battle against the Danes, and utterly defeated them.
His patriotism and personal sacrifice brought the clans together, and for the first (and arguably the last) time in history, all of Ireland was united under a native leader. At the age of 89, he was killed at the Battle of Clontarf. His army won the battle, but Boru, his son, and his grandson were all dead, thus the bloodline was ended. The clans soon took to quarrelling and by the time the next group of invaders came along, the Irish were once again a divided people.