Ferdiad was a warrior of Ulster's rival nation Connacht. Seeking the training of Scáthach, he traveled to the Land of Shadows. He met fellow student Cú Chulainn during his training, and they joined in a vow of brotherhood while competing for the right to obtain Gáe Bolg. Once Aife of the neighboring country attacked, Ferdiad, Cú Chulainn, and Scáthach all fought. Cú Chulainn captured Aife after a duel.
Once they were done with their training, both Ferdiad and Cú Chulainn left the Land of Shadows on the same day. Each asked the other if they would join their respective country instead of parting ways, but they both laughed upon realizing that it was not an option for either of them. Later upon Medb declaring war on Ulster, Cú Chulainn was the only combatant due to Ulster's curse. Due to Cú Chulainn killing her forces indiscriminately, Medb eventually agreed to one on one battles.
As Cú Chulainn proceeded to defeat each fighter, Medb eventually sent Ferdiad to fight him, but it was not of his own will. Equal in battle, Cú Chulainn eventually was forced to use Gáe Bolg to pierce his heart. His last farewell to Cú Chulainn was, "Gáe Bolg is a glory given to only the most gifted of warriors. In that brilliant hall of learning, you were our pride."
Ferdiad has a close relationship with Cú Chulainn, and Cú Chulainn treats him like an older brother.
Ferdiad is Cú Chulainn's equal in battle.