Brian Boru -- Emperor of the Irish

Over her exceptionally long life, Medb hErenn made many attempts to unify Ireland under a single king to make it an equal competitor among the nations of Europe and a rival to the growing power of England. Her last attempt was when she backed Brian Bóruma, known as Brian Boru, the king of the province of Munster, to become Ard Ri -- High King -- over all the island.

Up until then, the position of high king had been ceremonial; a sign of respect, but holding no real power without the cooperation of the provincial kings. Though he was not the first high king to try to change this, Brian came the closest, and in fact sought to create an entirely new position: Imperator Scottorum, Emperor of the Irish. By this means he hoped to reduce the power of the provincial kings and finally eliminate it altogether.

Through a series of conquests, alliances, and political maneuverings he succeeded in becoming High King, but revolts soon broke out, and it was during one these that Brian and his son and grandson were killed at the Battle of Clontarf outside of Dublin. Though technically a victory for Brian, with his death and the destruction of his dynasty the high kingship passed back to the family that had held it before him, a family that preferred the old way of a weak high king.

After Clontarf, Brian's greatest legacy was the founding of the Ui Briain clan, the O'Briens.

For more information, see this article.