Which king broke his geasa in 'The Destruction of Da Derga's Hostel'?

The king who broke his geasa in “The Destruction of Da Derga’s Hostel” was Conaire Mór.

Conaire Mór is a legendary High King of Ireland whose fate is told in the Old Irish saga Togail Bruidne Dá Derga. His geasa were sacred obligations or taboos imposed on his reign. They were not merely personal rules: breaking them signaled that the king’s relationship with the supernatural order was unraveling.

Conaire breaks several geasa before reaching Da Derga’s Hostel. One crucial taboo forbids him from being preceded by three red men into the house of a red man. As he approaches the hostel, he sees exactly that: three red riders entering the dwelling of Dá Derga, whose name means “Red God.”

The hostel is then attacked by Conaire’s foster-brothers and their allies. Conaire fights heroically, killing hundreds, but a magical thirst overwhelms him after the attackers exhaust the water supply while trying to burn the building. Mac Cécht eventually returns with water, but Conaire has already been mortally wounded. His severed head drinks and praises his champion.