Brian Boru (Bóruma) was born Brian mac Cennétig, to a small family of the Dál Cais tribe, in Munster. His mother and the mother of Conor (King of Connacht) were sisters. Brian Boru was seen as a young upstart by many of the Gaelic Chieftains, but later became the Ard-Rí (High King) of Ireland, and was the first non-O'Neill to sit in this position since Niall of the Nine Hostages.
Brian's brother, Mahon, had become King of Munster on the death of their father, Cennétig, in 951. Mahon and Brian fought against the Norsemen, who had imposed stiff taxes upon Munster, and in 975 Mahon was murdered by Ostermen (Norse). The King of the Ostermen of Limerick, King Ímar, fled to a monastary on Scattary Island. Brian followed him to the monastery and killed Ímar (Ivar in some texts) and his sons in revenge for his brother's murder. Some sources say that Brian killed Imar in battle, and others say he simply slaughtered him on Scattary Island. But either way, Brian killed Imar, and from that point Brian held both Munster and Limerick. He then made his way to Connacht and Leinster and, in 997, he met with Mael Sechnaill II (successor to Domnall UíNéill, King of Tara) and between the two of them they divided the whole of Ireland. Brian Boru and Mael Sechnaill met at Clontarf, which was later to be the place of Brian's death.
In 999 A.D., the people of Leinster and Dublin (primarily Norse) decided to go up against Brian, but were soundly defeated at Glenn Máma, and the King of Leinster was taken captive by Brian. The King of Dublin, Sitric Silkenbeard, was trounced later that year. In 1002 Brian demanded that Mael Sechnaill recognize him as King of Ireland, and Mael decided that he would (perhaps because many in Mael's kingdom saw Brian as a hero, restoring Ireland to greatness after the Viking invasions). Thus, Brian became High King of Ireland, ending the dynastic rule of the UíNéill (sons of Niall of the Nine Hostages) family, which had ruled for 600 years. He earned his name Brian Boru (Brian of the Tributes) by collecting tributes from the petty kings of Ireland, and used the monies to rebuild several monasteries and to restore the libraries which had been burned by the Osterman. Brian Boru is described in the Book of Armagh as the Emperor of the Irish, though he had to pay 20 ounces of gold, and recognize Armagh as the 'apostolic city of Ireland' to be listed as such.
In 1014 A.D., the Battle of Clontarf was waged between the Munstermen and the Ostermen. Munster won the day, but lost Brian in that battle. Despite the fact that Brian Boru was not backed by the majority of the men of Connacht, he and the men of Munster were victorious, and the Ostmen (Norsemen) were thereafter considered subordinate to the Kings of Ireland, and were no longer a military threat. They were relegated to their cities in Dublin, Waterford, Limerick, Wexford, and Cork, and retained some degree of autonomy there. After a time, the Norsemen married with the Irish, and, in effect, became Irish. Brian Boru, however, was killed in the final stages of the Battle of Clontarf.
With regard to Brian Boru, it is good to put the Battle of Clontarf into perspective. First, although it was a victory (which even the Vikings credit) to Brian's forces, the Vikings nonetheless kept their towns in Dublin, Wexford, Cork, Waterford and Limerick. These towns, cut off from each other by land, operated thenceforth as independent republics until finally assimilated into Irish culture.
Second, the Vikings were not defeated by a combined Irish force. The Vikings under the King of Dublin were supporting their Irish allies: the men of north Leinster. The Leinstermen perhaps suffered the worst defeat at Clontarf, since they lost their King, Ma'el Mo'rda. There were Vikings fighting in support of Brian, as well, from Limerick and Waterford. Also, many Irish forces stayed neutral, including Ma'el Sechnaill, whom Brian had usurped from the Ard Ri', and who wholly kept out of the battle. It was more a victory for Munster than anything else.
Finally, the attempt by Brian Boru to found a dynasty was itself defeated at Clontarf. He and his most promising
son fell at the battle. His remaining two sons, Tadg and Donnchad, were not strong enough to take the high
kingship, so it fell back into the grasp of Ma'el Sechnaill. After his death, in 1022, the Ard Ri' became more
honorary than a seat of true power. Brian's lasting impact was to set the precedent that the title belonged to
whoever could seize it, which was the case for the next 150 years.