The central tale of the Ulster Cycle is the Cattle Raid of Cooley (Táin Bó Cúailnge), which tells of the heroic single-handed defense of Ulster by the young Cú Chulainn. The men of Ireland, led by Ailill and Medb, attack Ulster in order to obtain the Brown Bull of Cúailnge (Cooley peninsula, Co. Louth). Cú Chulainn fends them off by engaging them in single combat, tragically slaying his beloved foster-brother Fer Diad in the process. The Bull is carried off and dies fighting against the White-Horned Bull (Finnbennach) of Connacht. A number of other tales, called foretales (remscéla), purport to explain the events that lead up to the Cattle Raid, although the connection between the foretales and the Cattle Raid is often tenuous. The reason for the inability of the Ulaid to defend themselves is given in the tale Ces Ulad “the debility of the Ulaid.” The otherworld woman Macha is forced to race against the king’s horses while heavily pregnant. She gives birth to twins on winning the race, and as she lies dying she curses the Ulstermen so that they will suffer the pangs of childbirth at times of greatest danger. The origin of the two bulls is explained in De Chophur in dá Muccida (“Of the generation of the two swineherds”). The swineherds of the title transform themselves into various animals to demonstrate their magical powers. When they take on the form of worms, they are swallowed by two cows that subsequently give birth to the two bulls. Another important prefatory tale is “The Exile of the Sons of Uisnech” (Longas macc nUisnig), which explains how various Ulster warriors, most notably Fergus mac Róich, went into exile in Connacht and so accompany Ailill and Medb on the Cattle Raid.
The earliest surviving version of the tale was compiled in the eleventh century from ninth-century material, and the earliest copy is preserved in Lebor na hUidre. This version has been heavily criticized for the lack of unity that results from the presence of different linguistic strata, doublets, variants, inconsistencies, and interpolations. However, the aim of the redactor was scholarly rather literary, and it has been suggested that he deliberately juxtaposed contradictory versions in an attempt to establish the historical facts. In the twelfth century, the tale was revised to produce a more consistent narrative, and this version is found in the Book of Leinster. The story was clearly known long before this, as it is referred to in three seventh-century poems: one attributed to the Morrígain, which is preserved in the Cattle Raid, Verba Scáthaige (“Scáthach’s words”), and a poem by Luccreth moccu Chérai. A later tradition attributes the “finding” of the story to the son of the seventh-century poet, Senchán Torpéist, who supposedly obtained it directly from Fergus mac Róich.