After this hostile act Flann in an alliance with Cathal mac Conchobair, king of Connacht, led an army into Leinster in order to help his son-in-law.

Despite the men of Munster being outnumbered and peace talks being held between both sides, Cormac refused to surrender, it ultimately proved to be his undoing, he and Cellach mac Cerbaill were killed during the course of the battle.

[3] Already with a distinct numerical disadvantage the Fragmentary Annals say that "the men of Munster came to the battle weak and in disorder" and they quickly broke and fled the field.

The contemporary poet Dallán mac Móre created the following poem which laments those killed, it has been preserved in Annals of the Four Masters: Cormac of Feimhin, Fogartach, Colman, Ceallach of the hard conflicts, they perished with many thousands in the great battle of Bealach-Mughna.

Flann of Teamhair, of the plain of Tailltin, Cearbhall of Carman without fail, On the seventh of the Calends of September, gained the battle of which hundreds were joyful.