Tadg, son of Nuada, was a druid and the maternal grandfather of Fionn Mac Cumhail in the Fenian Cycle of Irish mythology.
But Cumhal abducted Muirne, so Tadg appealed to the High King, Conn of the Hundred Battles, who outlawed and pursued him.
Cumhal was killed in the Battle of Cnucha at the hand of Goll mac Morna, who took over leadership of the Fianna, but Muirne was already pregnant.
Tadg rejected her and ordered his people to burn her, but Conn prevented this and sent Muirne away into the protection of an ally.
Muirne's son, Fionn, was born and brought up in secret, and when he grew up he took the leadership of the Fianna from Goll, and demanded satisfaction for his father's death from Tadg, threatening war or single combat if he was refused.