In Ireland, the battle came to be seen as an event that freed the Irish from foreign domination, and Brian was hailed as a national hero.

[2] The Vikings (or Norsemen) began carrying out raids on Gaelic Ireland in the late eighth century, and over the following few decades they founded a number of settlements along the coast.

[3] During the tenth century, Viking Dublin developed into the Kingdom of Dublin—a thriving town and a large area of the surrounding countryside, whose rulers controlled extensive territories in the Irish Sea and, at one time, York.

[8] In the tenth century, the Dál gCais, until then a small kingdom in what is now County Clare, began to expand.

[9] Mathgamain's brother, Brian Boru, quickly asserted his claim to the kingship of Munster, then invaded Leinster and gained its submission.

[11] He marched on Tara in 1000 with the combined armies of Munster, Osraige, Leinster, and Dublin, but after an advance party consisting of the latter two groups was destroyed by Máel Sechnaill, Brian Boru withdrew from the area without giving battle.

He completed the task when, following "a great hosting...by land and sea" into the Uí Néill territory of Cenél Conaill in 1011, the King was brought south to Dál gCais territory to submit to Brian Boru in person at his royal site of Cenn Corad.

[17] Sigtrygg went overseas in search of Viking support and enlisted the help of Sigurd Hlodvirsson, the Earl of Orkney and Brodir, a warrior of the Isle of Man.

Among the fallen on Brian's side, they name the High King himself, his son Murchad, and his grandson Toirdelbach, as well as his nephew Conaing, Domnall mac Diarmata of Corcu Baiscind (County Clare), Mac Bethad mac Muiredaig of Ciarraige Luachra (County Kerry), Mael Ruanaidh Ua hEidhin of Uí Fiachrach Aidhne, and Tadhg Ua Cellaigh of Uí Maine (both in south Connacht).

[22] No notables from Meath are recorded among the slain; leading to the suggestion that, if present, Máel Sechnaill kept himself and his forces out of harm's way.

[23] On the other hand, Cogad Gáedel re Gallaib ("The War of the Irish with the Foreigners"), says that the men of Meath came to the muster with Brian, but "were not faithful to him".

[24] According to the Cogad, after his arrival at Dublin, Brian sent his forces north across the river to plunder the area known as Fine Gall, and they torched the country as far as Howth.

To one side of them were Brian's Viking allies; Fergal ua Ruairc, with the Uí Briúin and the Conmhaícne was placed on the left flank.

The two men marched out into the middle of the field and fought, and both died, "with the sword of each through the heart of the other, and the hair of each in the clenched hand of the other.

According to the Cogad Gáedel re Gallaib, Brian was visited by a Banshee-like spirit named Aibhill prior to the battle who warned him of his impending demise.

There it was met by the coarb of Patrick, the traditional head of the church in Ireland, who brought the body back with him to Armagh, where it was interred after twelve days of mourning.

According to the annals, Donnchad rallied the forces of the Dál gCais at Clontarf and led them home to Cenn Corad.

In 1052, Diarmait mac Máel na mBó, King of Leinster, captured Dublin and Fine Gall, for the first time asserting Irish overlordship over the Norse of Ireland.

[41] In modern times there has been a long-running debate among historians, which is now 250 years old, about Ireland's Viking age and the Battle of Clontarf.

However revisionist historians see it as an Irish civil war in which Brian Boru's Munster and its allies defeated Leinster and Dublin, and that there were Vikings fighting on both sides.

[42][43] In January 2018, researchers from the Universities of Coventry, Oxford and Sheffield, led by Coventry University theoretical physicist Professor Ralph Kenna, published a paper[44] in the journal Royal Society Open Science, that used network science to mathematically analyse a medieval text, Cogadh Gáedhel re Gallaibh (The War of the Gaedhil with the Gaill, meaning invasions of Ireland by the Danes and other Norsemen), that listed over 1000 relationships between about 300 characters, and concluded that the standard and popular view was broadly correct, but that the picture was nevertheless more complex than "a fully 'clear-cut' Irish versus Viking conflict".

[42][43] However one of the paper's co-authors,[44] PhD student Joseph Yose, added that "Our statistical analysis ... cannot decisively resolve the debate".