Fergus mac Róich

Conchobar's intended bride, Deirdre, elopes with the young warrior Naoise and his two brothers, and after some time of wandering they are tracked down to an island off Scotland.

When they arrive, Fíachu, Naoise and his brothers are murdered by Éogan mac Durthacht, a former enemy of Conchobar who has recently made peace with him.

After hearing of this betrayal, Fergus, Cormac and Dubthach burn Emain Macha and defect to Connacht with three thousand followers, where they enter the service of king Ailill mac Máta and queen Medb.

[5] Flidais, wife of Ailill Finn, a petty king of the Erris district of Connacht, falls in love with Fergus from afar.

However, he proves difficult to defeat, beating Dubthach in single combat and holding out against a siege of his stronghold.

[7][8] When Ailill and Medb raise a huge army and launch the Táin Bó Cúailnge (Cattle Raid of Cooley) to steal Ulster's stud bull, Donn Cúailnge, Fergus' knowledge of the terrain means he is chosen to lead the way.

The Connacht army is routed, and Fergus bitterly remarks, "It is the usual thing for a herd led by a mare to be strayed and destroyed.

[10] One day, after Fergus has been in exile for fourteen years, Ailill sees him swimming in a lake with Medb, and is overcome with jealousy.

He tells his brother, Lugaid Dalleces, who is blind, that deer are playing in the water, and persuades him to throw a spear at them.

[14] The Ciarraige, an early medieval people who gave their name to County Kerry, traced their ancestry to Ciar, a son of Fergus and Medb, as do the Conmhaícne of Leitrim, Sligo, and Galway, through Conmac.

The Masraige, a tribe inhabiting Magh Slécht in County Cavan were descended from Fergus according to Corpus Genealogiarum Hiberniae, 1 16r A44, 279.