The war started when Cormac mac Cuilennáin and an adviser Flaithbertach mac Inmainén assembled an army to campaign against their eastern neighbours, Leinster, whose king Cerball mac Muirecáin was Flann Sinna's son-in-law and staunch ally. The Fragmentary Annals of Ireland, a source compiled in the 11th century for Donnchad mac Gilla Pátraic, king of Osraige, and king of Leinster, contain a long account of these events, perhaps written within living memory. Before the battle many men had deserted Cormac, because while riding through the camp Flaithbertach's horse stumbled and threw him to the ground. This, it is said, was taken to be a very bad omen. Many of the Munstermen were unwilling to fight, and news of this came to Cerball mac Muirecáin, who proposed a negotiated settlement. The Leinstermen would pay tribute, and give hostages, but the hostages would be given to Móenachm abbot of Diseart Díarmata, rather than to the Munstermen. Cormac, it is said, was willing to accept this settlement, but Flaithbertach—Byrne refers to him as "Cormac's evil genius"— was not and persuaded Cormac to fight, in spite of the king's conviction that he would be killed.
Battle
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. Many were killed. Cormac was among them, after his neck was broken from falling off his horse while fleeing with his men, Flaithbertach his adviser was captured. Cormac was beheaded and his head taken to Flann Sinna. The Fragmentary Annals say:
"That is indeed evil," said Flann to them, and it was not thanks that he gave them. "It was an evil deed," he said, "to cut off the holy bishop's head; I shall honour it, and not crush it." Flann took the head in his hands, and kissed it, and he carried the consecrated head and the true martyr around him three times.
Aftermath
After the battle Cormac was considered a saint, his death left Munster without its own king until about 914 when Flaithbertach mac Inmainén was chosen. 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. The bishop, the souls' director, the renowned, illustrious doctor, King ofCaiseal, King of Iarmumha; O God! alas for Cormac!
