The Ulster Cycle stories are set in and around the reign of King Conchobar mac Nessa, who rules the Ulaid from Emain Macha (now Navan Fort near Armagh). The most prominent hero of the cycle is Conchobar's nephew, Cú Chulainn. The Ulaid are most often in conflict with the Connachta, led by their queen, Medb, her husband, Ailill, and their ally Fergus mac Róich, a former king of the Ulaid in exile. The longest and most important story of the cycle is the Táin Bó Cúailnge or "Cattle Raid of Cooley", in which Medb raises an enormous army to invade the Cooley peninsula and steal the Ulaid's prize bull, Donn Cúailnge, opposed only by the seventeen-year-old Cú Chulainn. In the Mayo Táin, the Táin Bó Flidhais it is a white cow known as the 'Maol' that is the object of desire, for she can give enough milk at one milking to feed an army. Perhaps the best known story is the tragedy of Deirdre, source of plays by W. B. Yeats and J. M. Synge. Other stories tell of the births, courtships and deaths of the characters and of the conflicts between them.
The stories are written in Old and Middle Irish, mostly in prose, interspersed with occasional verse passages. They are preserved in manuscripts of the 12th to 15th centuries but, in many cases, are much older. The language of the earliest stories is dateable to the 8th century, and events and characters are referred to in poems dating to the 7th. The tone is terse, violent, sometimes comic, and mostly realistic, although supernatural elements intrude from time to time. Cú Chulainn in particular has superhuman fighting skills, the result of his semi-divine ancestry, and when particularly aroused his battle frenzy or ríastrad transforms him into an unrecognisable monster who knows neither friend nor foe. Evident deities like Lugh, the Morrígan, Aengus and Midir also make occasional appearances.
Unlike the majority of early Irish historical tradition, which presents ancient Ireland as largely united under a succession of High Kings, the stories of the Ulster Cycle depict a country with no effective central authority, divided into local and provincial kingdoms often at war with each other. The civilisation depicted is a pagan, pastoral one ruled by a warrior aristocracy. Bonds between aristocratic families are cemented by fosterage of each other's children. Wealth is reckoned in cattle. Warfare mainly takes the form of cattle raids, or single combats between champions at fords. The characters' actions are sometimes restricted by religious taboos known as geasa.
The events of the cycle are traditionally supposed to take place around the time of Christ. The stories of Conchobar's birth and death are synchronised with the birth and death of Christ, and the Lebor Gabála Érenn dates the Táin Bó Cúailnge and the birth and death of Cú Chulainn to the reign of the High King Conaire Mor, who it says was a contemporary of the Roman emperor Augustus (27 BC — AD 14). Some stories, including the Táin, refer to Cairbre Nia Fer as the king of Tara, implying that no High King is in place at the time.
The presence of the Connachta as the Ulaid's enemies is an apparent anachronism: the Connachta were traditionally said to have been the descendants of Conn Cétchathach, who is supposed to have lived several centuries later. Later stories use the name Cóiced Ol nEchmacht as an earlier name for the province of Connacht to get around this problem. However, the chronology of early Irish historical tradition is an artificial attempt by Christian monks to synchronise native traditions with classical and biblical history, and it is possible that historical wars between the Ulaid and the Connachta have been chronologically misplaced.