On 13 September 908, Flann, aided by his son-in-law Cerball mac Muirecáin, and Cathal mac Conchobair, King of Connacht, fought against the Munstermen, again led by Cormac and Flaithbertach, at the Battle of Bellaghmoon (near Castledermot, County Kildare).
In 910, now without the aid of Cerball, who had died of sickness, Flann defeated the men of Bréifne.