Brian Boru (Brian mac Cenneidigh) had ruled most of Ireland since 1002, but the island was still highly fractional and the title of "high king" had been largely ceremonial. Brian looked to change this, and unite the island, which he set about doing over a period of years.
In 1012 the king of Leinster, Mael Morda, rose in revolt. His attempts were quickly thwarted when Brian arranged a series of cross-marriages, giving his daughter to Sigtrygg Silkbeard, leader of the Dublin Vikings, himself marrying Sigtrygg's mother and Mael's sister, Gormlaith. However this alliance was destined not to last, and in 1013 Mael again went to the Sigtrygg after being admonished by Gormlaith for accepting Brian's rule. This time Sigtrygg was ready to fight, and various Irish clans who were envious of Brian quickly joined.
Brian immediately threw Gormlaith in jail, and went on a series of raids around Dublin in order to tie down any Irish who would attempt to join the viking forces. Meanwhile Gormlaith contacted Sigurd, the viking earl of the Orkney Isles, to come to her aid. He not only agreed, but in turn contacted Brodir of the Isle of Man to join the fight. Sigurd and Brodir both planned on killing the other after the battle to take the seat of High King for themselves, while Sigtrygg was busy trying to form alliances with everyone involved in an attempt to at least retain his own seat in Dublin.
In 1014 Brian's army had mustered and set off towards Dublin. As they approached, the Irishmen of Meath, commanded by ex-high king Malachi, refused to take part in the battle. This left him with 7,000 men, outnumbering the 2,000 or so under Sigtrygg, but considerably worse equipped in comparison. They arrived outside the walls of Dublin and set up camp.
That night Brian received news that the Viking forces had boarded their longships and headed out to sea, deserting Sigtrygg. This was in fact a ruse. After nightfall they turned around and landed on the beaches of Clontarf, just over a mile to the north of Dublin, in order to surprise Brian's army the next day. At the time Dublin was only on the south shore of the Liffey River, connected to the north bank, and Clontarf, only by a single bridge. This allowed the vikings time to disembark and prepare in relative safety.
The Viking army formed up into five divisions on the field, while Sigtrygg and 1,000 of his men remained in town. Sigtrygg's son commanded the extreme left of the line with 1,000 of the men from Dublin that decided to fight in the open. Mael Morda added another 3,000 men from Leinster in two divisions. Although numerous, they too were poorly armed in comparison to the vikings on either side. Sigurd's Orkney Vikings manned the center with 1,000 men, and Brodir's Vikings added another 1,000 or more on the right, on the beaches.
Brian's forces were arranged in a similar fashion. On the right (the viking left) were 1,000 foreign mercenaries and Manx Vikings. Next to them, 1,500 clansmen of Connacht were gathered under their kings, while more than 2,000 Munster warriors under Brian's son Murchad continued the front, flanked by 1,400 Dal Caissans on the extreme left led by Murchad's 15-year-old son, Tordhelbach, and Brian's brother, Cuduiligh. Off to the right and several hundred yards to the rear stood Malachi's 1,500 men who simply watched.
The battle opened with several personal taunts between men in either line, often ending with the two men marching out into the middle of the field to enter personal battle, while the forces on either side cheered. While this went on the two groups slowly edged towards each other. They engaged early in the morning.
At first the battle went the vikings way, with their heavier weapons prevailing over their opponents as everyone had expected. This advantage also served Brian, who's Viking mercenaries on his right slowly pushed back the forces facing them. On the left, Brodir himself led the charge and gained ground, until he met a local warrior, Wolf the Quarrelsom. Although Wolf was unable to break Brodir's armor, he knocked him to the ground and Brodir fled to hide. This left the now leaderless Viking force facing Murchad's forces, who considered themselves the "kings own" (containing many of Brian's more distant relatives) and by the afternoon Brodir's forces were fleeing to their ships.
In the center things were going more the Viking's way. Both Sigurd's and Morda's forces were hammering into the Leinster forces. However Sigurd carried a "magical" standard into battle which drew the Irish warriors to it, eventually forcing their way in and killing the bearer. Although the standard was supposed to guarantee a victory for the bearer's forces, it also guaranteed the bearer's death. Fearing it, no one else would pick it up, so Sigurd did and was quickly killed.
By the end of the day, after several mutual pauses for rest, the Vikings found themselves with both flanks failing, Sigurd dead, and everyone exhausted. The beaches in front of the ships were already lost, and many men took to trying to swim to the ships further offshore, drowning in the process. The battle was now clearly going Brian's way, and the Dublin Vikings decided to flee to the town. At this point Malachi decided to re-enter the battle, and cut them off from the bridge. The result was a rout, with every "invading" Viking leader being killed in the battle.
Meanwhile Brodir, hiding in the woods near Dublin, noticed Brian praying in his tent. Gathering several followers they ran into the tent and killed him and his retainers. Then they retreated, with Brodir yelling, Now let man tell man that Brodir felled Brian. He was later captured, and according to Viking accounts, killed.
Of the 7,000 to 8,000 Vikings and allied forces, an estimated 6,000, including almost all the leaders, were killed. Irish losses were least 1,600, and perhaps as high as 4,000, including their king and most of his sons. There was no longer any clear line of succession.
With the Irish now leaderless, and the power of the Vikings as a political force broken, Ireland soon returned to a series of bloody fractional fighting. However things had changed as a result of the battle, with Viking and Gael culture no longer contesting power. After a number of years this led to a lasting peace, and the Vikings would instead turn to England and Scotland, eventually taking power when Canute was installed as King in 1015.
Sigtrygg had watched the battle with Gormlaith from Dublin, and with the Irish army melting away the next day, ended up perhaps the only "winner" of the contest, leading in Dublin until his death in 1042.