Fergus son of Roach, a key figure in the Ulster cycle, was foster-father of Cú Chulainn (qv), lover of Queen Medb (qv), and (as mac roich ‘son of the great horse’) the most virile man in Ireland. He was more than a literary character, however, for besides being a powerful warrior and former king of Ulster he was also legendary ancestor to several historic Irish peoples, including the Ciarraige who give their name to modern Co. Kerry. Fergus's positions within the literary and genealogical traditions were linked, particularly in tales about the finding of Táin Bó Cuailnge, the pre-eminent Irish epic. These tell how the Irish discovered that they no longer possessed a full account of the Táin because they had traded it for Isidore of Seville's encyclopaedic Etymologiae. They come to realise that their greatest tale can only be recovered through summoning Fergus from the dead, a task aided in one version by invoking the saints who are descended from him. This summoning is successful and he restores the Táin to knowledge. Indeed, Fergus is a particularly apposite narrator for he was one of the epic's protagonists, the man who sums up its misogynist themes through famously suggesting, in the penultimate scene, that all herds led by mares are doomed to destruction, in the same way that the female leadership of Queen Medb had brought disaster to all her followers.
Fergus is a contradictory figure. He is a king who loses his kingship through a ruse because he so desires the ambitious and wily Ness as a sexual partner. She manipulates Fergus's lack of judgement to ensure that her own son, Conchobar (qv), becomes king of Ulster in his place. Fergus is a man noted for his honour and adherence to the warrior code, but he finds that his guarantees to the sons of Uisliu are betrayed by King Conchobar in the tragic saga Longes mac nUislenn. Their murder leads to his exile from Ulster. Fergus is admired for his virility and yet he is symbolically castrated through having an adulterous affair with Queen Medb during the Táin, one detected by her husband Ailill, who steals Fergus's sword in revenge, greatly weakening him as a warrior.
Indeed, Fergus’ death-tale, Aided Fergusa meic Roich, sums up many of these contradictions. The short narrative describes how the sexual relationship between Medb and Fergus continues, despite her defeat in the Táin. The moment of disaster strikes when the two lovers engage in an overtly erotic swim in a lake while Ailill looks on from the shore. Medb's husband, finally overcome by jealousy, tricks his blind brother Lugaid into throwing a spear at Fergus, killing him. Lugaid, although blind, was famed for the accuracy of his spear-casts. He is distraught when he learns that his target was Fergus for they are foster-brothers, a relationship of enormous importance to the writers of the Irish sagas. Ultimately, it seems that Fergus as a hero was fatally vulnerable to trickery. He loses his kingship through a trick; he leaves Ulster as an exile, dishonoured by Conchobar's treachery and trickery; he is murdered as the direct result of a trick played by King Ailill upon his brother. Perhaps the writers of the early Irish sagas are suggesting that while Fergus had the physical attributes of an ideal king, he lacked the mental agility and wiliness which would have brought him greater success.