From Walsh's History of the Irish Hierarchy: With the Monasteries of Each County, Biographical Notices of the Irish Saints, Prelates, and Religious, 1854, c. xliv, pps. 420ff:
Clontarf is in the barony of Coolock and at the mouth of the river Liffey. The Danes were defeated at Clontarf and their power annihilated by the victory which Brian Borumhe gained over them on Good Friday the 23d of April 1014.
On that day the pious monarch of Ireland would have avoided fighting but left no alternative as the Danes insisted he resolved on the defence of the rights of his country and religion. Holding a crucifix in his left hand and a sword in his right the monarch rode through the ranks with his son Moragh encouraging his army to terminate forever the oppressions of those tyrants and usurpers who had committed so many cruelties and sacrileges in Ireland so that the memorable day on which Christ shed his blood on the altar of the cross in expiation of our sins should be the last of their power in the kingdom and declaring his readiness to sacrifice his life in so holy and righteous a cause.
As soon as the engagement began Maelseachlin with his men of Meath withdrew and continued as mere spectators of the battle Notwithstanding their inactivity and defection Brian and his faithful troops who heroically fought from sunrise until the close of the day gained a complete victory which shall be ever memorable in the annals of Ireland.
According to one account the Ostmen or Danes between killed and wounded lost thirteen thousand men and the people of Leinster who joined the Danes three thousand. The thousand Danes who wore coats of mail are said to have been cut to pieces with their leaders Charles and Henry Dolat and Conmaol. Among the slain were also Brodar and two Danish princes of Dublin with Maelmurry king of Leinster.
The victory however was dearly purchased for besides a great number of the Irish forces Brian the monarch Morogh his son and Turlogh his grandson fell in this memorable contest together with many chieftains of Munster and Connaught. The monarch was slain in the 88th year of his age and Morogh in the 63d.