Naoise

Naoise elopes with Deirdre after she chooses him over the aging King Conchobar mac Nessa, to whom she was promised at birth under a prophecy that she would bring ruin to Ulster. He flees with her and his two brothers to Alba, living in exile for years until Conchobar sends Fergus mac Róich to guarantee their safe return.

That guarantee is broken: Conchobar has Naoise and his brothers killed shortly after they land back in Ulster, an act of betrayal that drives Fergus into open rebellion against his own king. Naoise's death is the hinge the whole tragedy turns on — Deirdre never accepts Conchobar afterward, and the Longes mac nUislenn closes with her own death rather than submission to him.