Father of the Fenian hero Diarmait Ua Duibne, whose vengeance causes his son to be pursued by a magical boar. Diarmait's mother, Cochrann (also Cróchnat), has committed adultery with a shepherd, bearing another son. Angus Óg also raises this son, a half-brother. Perceiving this bastard child as a reproach, Donn kills him by crushing him between his legs. The shepherd father touches the corpse with a hazel wand that revives him, and changes him into a boar who follows Diarmait until his dying day.