One of the greatest of all Ulster heroes, best known for (a) being tutor to Cúchulainn; (b) losing his throne to Conchobar mac Nessa through the treachery of Ness; (c) encouraging Deirdre and Noíse to return to Ireland; (d) going into exile to join Queen Medb's forces in Connacht; and (e) revealing the story of the Táin Bó Cuailnge [Cattle Raid of Cooley] to the poet Senchán Torpéist. Additionally, Fergus is characterized from his earliest portrayals as having great sexual energy, as implied in his earliest patronymic, Roach, possibly from Ro-ech [i.e. great horse]. He had huge genitalia, requiring seven women to satisfy him. Fál, the upright stone at Tara, was known in 19th-century oral tradition as Bod Fhearghais [Ir., Fergus's penis], perhaps implying Fergus mac Róich. His great sword is Caladbolg.
Most of our perception of Fergus's persona is drawn from his description in the Táin, augmented by both earlier and later texts. Fergus is king of Ulster, resident at Emain Macha, when he falls in love with Ness, the daughter of Eochaid Sálbuide. She agrees to marry Fergus but only on condition that her son from a previous encounter, Conchobar mac Nessa, be allowed to become king for a year, almost as a trial. Fergus agrees to this, becoming also the foster-father of Conchobar, but Ness contrives with the nobles to prevent Fergus's return. In texts outside the Táin, an ambitious young Conchobar drives Fergus from Emain Macha, whence the older man allies himself with Tara, leading an unsuccessful rebellion against his former kingdom.
Fergus encourages the sons of Uisnech, Noíse, Ardan, and Ainnle, to return with Deirdre from Scotland, whence they had fled because Conchobar had desired her, giving his honour as a pledge for their safety. Once the four are back in Ulster, he departs for a three-day banquet, under an obligation to a geis put upon him. In Fergus's absence Conchobar, who has vouched for the lovers' safety, has the sons of Uisnech murdered and takes Deirdre, at last, for himself. Outraged at this dishonour, Fergus returns and burns Emain Macha to the ground. He and his warriors then depart for Cruachain, where they join forces with Medb and her husband, Ailill of Connacht, with whom he serves during most of the action of the Táin. He gives information about his former subjects, but is sometimes reluctant to oppose them in battle, once warning them of his arrival and leading his own forces on a detour south to give the Ulstermen time to assemble. None the less, he kills hundreds of Ulstermen with Caladbolg and would have slain Conchobar, if he had not been prevented from doing so by Cormac Connloinges, Conchobar's son. To vent his rage and disappointment he is said to have struck off the tops of three hills in Co. Meath.
Several stories outside the Táin link Fergus and Medb. In an obscure 7th-century verse Fergus is portrayed as deserting his own people because of his attachment to Medb. They are often seen as lovers, as in the best-known story of Fergus's death. When they are swimming together one day, the nude queen is sitting on his breast with her legs entwined around him. Medb's husband Ailill, understandably jealous, remarks ironically, ‘It is delightful to see what the hart and doe are doing in the lake.’ Hearing this, Lugaid (1), a warrior-poet noted for his accuracy, despite his blindness, understands the allusion to hunting. He throws his lance so that it passes through the heart of Fergus and exits through his back. In the 7th-century version, Fergus dies near Larne, Co. Antrim, and thus the town of Carrickfergus is named for him. In the 10th- or 11th-century version, the lake is Finnloch (Lough Carrowmore, Co. Mayo). This swimming episode links Fergus mac Róich with the lesser-known Fergus mac Léti. The love-child who resulted from Fergus's union with Medb is Ciar, whose descendants give their name to the Ciarraige, whence Co. Kerry.