Fearghal was killed at the battle of Almhain (Allen, near Kildare), in 718.

In this engagement, the Leinster men only numbered nine thousand, while their opponents numbered twenty-one thousand.

The Leinster men, however, made up for numbers by their valour; and it is said that the intervention of a hermit, who reproached Fearghal with breaking the pacific promise of his predecessor, contributed to the defeat of the northern forces.
Another battle took place in 733, when Hugh Allan, King of Ireland, and Hugh, son of Colgan, King of Leinster, engaged in single combat.

The latter was slain, and the Leinster men "were killed, slaughtered, cut off, and dreadfully exterminated." In fact, the Leinster men endured so many "dreadful exterminations," that one almost marvels how any of their brave fellows were left for future feats of arms.

The "northerns were joyous after this victory, for they had wreaked their vengeance and their animosity upon the Leinster men," nine thousand of whom were slain.