In Irish mythology, Mesgegra (Mes Gegra, Mes Gedra) was king of Leinster during the events of the Ulster Cycle.
Conall Cernach killed Mesgegra in single combat following a battle provoked by the Ulster poet Athirne. Mesgegra had lost a hand in an earlier fight, so Conall fought him with one hand tucked into his belt. He put Mesgegra's head on his shoulder, and this straightened his neck. Conall's charioteer couldn't carry the head, so he cut out the brain and preserved it by mixing it with lime. The calcified brain was later stolen by Cet mac Mágach and used to kill Conchobar mac Nessa.