Cet mac Mágach is a Connacht warrior in the Ulster Cycle of Irish Mythology.
At a feast at the house of Mac Dá Thó, a hospitaller of Leinster, the warriors of Connacht and Ulster competed for the champion's portion by boasting of their deeds.
Cet admitted defeat, but claimed that if his brother Anlúan were present, his feats would top even Conall's.
He stole the calcified brain of Mesgegra, king of Leinster, which Conall Cernach had taken as a trophy of battle, and shot it from his sling, embedding it in Conchobar's head.
Cet made a raid on Ulster one winter's day, killing twenty-seven men and taking their heads.