Sometimes with the patronymic mac Máile Dúin, ‘son of Máel Dúin'. A king of Ailech better remembered in death than in life. After Fergal was defeated by a force of Leinstermen near the Hill of Allen, his head was severed, the hair washed, and combed, and set upon a pike. Shortly after this Badb, the battle-goddess in the form of a raven, hovered over Fergal's head and that of the youth Donn Bó, a youth celebrated for the sweetness of his song who had also been decapitated in battle. At the feasting by the victors that night, the severed head of Donn Bó began to sing the praises of Fergal.
From A Dictionary of Celtic Mythology in Oxford Reference.