O'Donnell rapidly came to power, initially with the support of the FitzGerald dynasty.
In 1257, he drove the Normans out of northern Connacht[1] after the battle of Creadran-Cille killing Maurice FitzGerald, 2nd Lord of Offaly, in personal combat, but suffering severe injuries.
[2] O'Donnell, while still incapacitated by his wound, was summoned by O'Neill to give hostages in token of submission.
Carried on a litter at the head of his clan, he gave battle to Brian, whom he defeated with severe loss in prisoners and cattle.
[1] O'Donnell died of his wound immediately afterwards outside of where the town of Letterkenny is today, and was succeeded in the chieftainship by his brother Donal Óg, who returned from Scotland in time to withstand successfully the demands of O'Neill.