In 908, Cormac and Flaithbertach mac Inmainén, Cormac's chief councillor and abbot of Scattery Island, collected an army to campaign against their eastern neighbours, Leinster, whose king Cerball mac Muirecáin was Flann Sinna's son-in-law and staunch ally.
Cormac was beheaded and his head taken to Flann Sinna.