The northern king appears to have brought south with him just a picked force of about one thousand men. Aed was faced with what was an alliance by the kings of greater Leinster against his hegemony – as they all feared dominance by a king as powerful and as skilful as he.
In the event he defeated his enemies in what was probably a hard fought battle. It looks like the men of the East were the ones who opened the battle but were held in check. As both sides had more or less equal numbers its probable that the contest was a long drawn out affair. It certainly was a bloody one as the King of Brega and his ally Diarmait mac Etarscéle, the King of Loch Gabor were amongst the fallen. Many of the Vikings were also put to sword. But King Aed lost from amongst his own one Fachtna mac Mael Dúin, (the man selected to succeed him in the North) when he launched his own counter assault upon his enemies.
King Aed was one of the most successful kings to rule in Ireland in the 9th Century. He drove the Vikings out of the North and checked their power in north Leinster. He was also adept at keeping Irish rivals from usurping him from his throne. He died a peaceful death at the Monastery of Dromiskin, some 30 miles north of where this great battle was fought, in the year 879.
Aed (the son of Niall) who was the king of Temair [Tara], and Conchobor (the son of Tadc) who was the king of Connacht, won a battle at Cell ua nDaigri on the 8th of the Ides [6th of September]against the Uí Néill of Brega and the Laigin, and a large force of foreigners, i.e. three hundred or more.
Flann son of Conaing had five thousand and Aed Finnliath had one thousand,
Flann son of Conaing, king of all Brega, and Diarmait son of Etarscéle, king of Loch Gabor, fell in this battle.
Very many of the heathens were slaughtered there and Fachtna (the son of Mael Dúin), heir designate of the North, fell in the counterattack of the battle.
Chronicon Scotorum 868 AD