The numbers of both sides probably added up to about 5,000 men and the battle was fought all day from morning to evening. By its end the rebels and vikings were defeated by the Munstermen and the jarl of Orkney was dead. King Brian's son Murchad, the commander of the Munster army was also fatally wounded and died the next morning. According to the Irish legend of the battle, the Cogadh Gaedhel re Gallaibh, Murchad's son Turlough was also killed, drowned as he fought hand-to-hand with some vikings at the weir of Clontarf.
Brian Boru is one of the great medieval Irish ancestor figures. O'Brien is today one of the most popular Irish surnames and it is thought that all holders of the name are descended from King Brian. In later medieval times the O'Brien family ruled Thomond or modern County Clare, but branches of the family spread out to settle in east Limerick, Ara in north Tipperary, and also in the Glen of Aherlow and the Comeragh mountains of Waterford. There are also many other Munster surnames associated with close members of King Brian's family. The O'Kennedys of Irish Ormond are said to be descended from Brian's brother Donnchuan, while the McMahons of west Clare are descended from an illustrious great-grandson of Brian who also became high-king of Ireland. Collectively all these families associated with Brian Boru are known as the Dál Cais.
The battle of Clontarf is one of my favourite episodes in Irish history and it remains an important event due to the extensive saga material relating to it in both the Irish and Scandinavian traditions. Unfortunately for the victorious Munster army as they gave chase to their fleeing enemies, one of the Viking warriors Brodir from Man fled in the opposite direction into a small wood. Once in hiding he saw the men guarding King Brian with a shieldburg around the king's tent, leave to join in the pursuit of the fleeing vikings and rebels. Brodir took his chance and attacked King Brian's tent, killing the elderly king who was guarded by only a few young servants. So Brian died right at his moment of victory. Below I have attached two versions of the death of King Brian, the first from the Irish saga Cogagh Gaedhel re Gallaibh,which was written in the early 1100s. The second is from the famous Scandinavian saga Njal's Saga, which was written in Iceland around the year 1280 AD. Brodir did not live long to enjoy his fame for killing Brian Boru. The Munster soldiers returned to aid their king and when they found him dead they captured Brodir and executed him.
While they were engaged in this conversation the attendant perceived a party of the foreigners approaching them. The Earl Brodir was there, and two warriors along with him. ‘There are people coming towards us here’, said the attendant. ‘Woe is me, what manner of people are they?’ asked Brian. ‘A blue stark naked people’, said the attendant. ‘Alas!’, said Brian, ‘they are the foreigners of the armour, and it is not to do good to thee they come’. While he was saying this, he arose and stepped off the cushion, and unsheathed his sword. Brodir passed him by and noticed him not. One of the three who were there, and who had been in Brian’s service said ‘King, King’, said he, ‘this is the King’. ‘No, no, but Priest, Priest’, said Brodir, ‘it is not he’, says he, ‘but a noble priest’. ‘By no means’, said the soldier, ‘that is the great king, Brian’. Brodir then turned round, and appeared with a bright, gleaming, trusty battle-axe in his hand, with the handle set in the middle of it. When Brian saw him he gazed at him, and gave him a stroke with his sword, and cut off his left leg at the knee, and his right leg at the foot. The foreigner dealt Brian a stroke which cleft his head utterly; and Brian killed the second man that was with Brodir, and they fell mutually by each other. 
Brodir saw that King Brian’s forces were chasing the fugitives and that there were only a few men at the shieldburg. He ran out of the woods and cut his way through the shieldburg and swung at the king. The boy Tadhg brought his arm up against it, but the blow cut off the arm and the king’s head too, and the king’s blood fell on the stump of the boy’s arm, and the stump healed at once.
Then Brodir called loudly, ‘Let word go from man to man – Brodir killed Brian’.
 Njal’s Saga, pp 301-03.
, pp 198-203.