Cormac Mac Airt is probably the most famous of the ancient kings of Ireland, and is now thought to have been an authentic historical king. Cormac was the son of Art, son of Conn of the Hundred Battles. He ruled Tara, the seat of the High Kings of Ireland, for forty years (probably 227 - 266 CE) and under his rule Tara flourished. He was famous for his wise, true, and generous judgments.

Cormac owned the wonderful gold cup given to him by the sea-god Manannan Mac Lir in the Land of the Living. If three lies were spoken over it, it would break in three; three truths made it whole again. Cormac used this cup during his kingship to distinguish falsehood from truth. When Cormac died, the cup vanished, just as Manannan had predicted it would.