Son of Domnaill Mor O Donnchadha, he notoriously launched a raid on the monastic Inishfallen:
‘There was committed in this year (1180) adeed which greatly vexed the clergy of all Ireland, namely, the plundering of Inis Faithlinn by Mael Duin…and the carrying off by him of all the worldly wealth therein…He
collected…gold, silver… mantles and cloaks…without any respect for God or man, but the mercy of God did not allow him to kill people or to strip this heavenly place of church furnishings or books.’

In defense of Mael Duin, Inisfallen at this time was allied with the Ua Briain faction and may have been a target for strategic and political and
not just avaricious intentions.