In childhood, this person's name was Deimne, and he gained his more famous nickname after his hmair turned prematurely white.
This man's second betrothed, Grainne, falls for one of his men, and runs away with her. He is the grandson of Tadg mac Nuadat, and his father kidnapped his mother, Muirne, which led to the Battle of Cnucha. He had two formerly human hounds, who recognized a formerly human deer, Sabdh, who became his wife. In childhood, this person's name was Deimne, and he gained his more famous nickname after his hmair turned prematurely white. He is the enemy of Goll MacMorna, who killed his father, and in one myth, he catches the Salmon of Knowledge for his teacher, Finnegas. For ten points, identify this father of Oisin, the hero of the Fenian cycle.
This man's second betrothed, Grainne, falls for one of his men, and runs away with her.
{Finn MacCool} (actually spelled “Fionn Mac Cumhaill”, so give them some leeway)