Mesgegra is a legendary figure in the Ulster Cycle of Irish mythology, identified as a King of Leinster. He is a primary character in the medieval Irish narrative Talland Étair (The Siege of Howth).
In the mythological tradition, Mesgegra’s reign was marked by a conflict with the province of Ulster, precipitated by the satirist Athirne. Following a siege at Howth, Mesgegra retreated but was pursued and intercepted by the Ulster warrior Conall Cernach at a ford. The two engaged in a duel in which Mesgegra was defeated and beheaded.
Mesgegra is particularly noted in Irish literature for the specific role his remains play in subsequent legends. According to the accounts, Conall Cernach extracted Mesgegra's brain, mixed it with lime, and allowed it to harden into a "brain ball"—a common mythological trope involving the preservation of a defeated enemy's power. This object was kept as a trophy in Emain Macha until it was stolen by the Connacht warrior Cet mac Mágach. Cet used the brain ball as a sling stone to strike King Conchobar mac Nessa of Ulster, an injury that eventually led to the king's death years later. This event is often interpreted within the cycle as Mesgegra achieving vengeance from beyond the grave.
Mesgegra’s wife, Mes Roeda, is also mentioned in the cycle; she is recorded as having died of grief on the same day as her husband. The site where she died was traditionally said to be marked by a burial mound known as Lorgue Mes Roeda.