The Ulster Cycle is a body of medieval Irish heroic legends and sagas which makes up one of the four great cycles of Irish mythology. It features the heroes of the Ulaid, the people who gave their name to the province of Ulster, in the reign of their king Conchobar mac Nessa at Emain Macha (now Navan Fort near Armagh), traditionally dated to the 1st century BC/1st century AD, and their enemies, particularly the Connachta under queen Medb and her husband Ailill. The longest and most important story is the Táin Bó Cúailnge or "Cattle Raid of Cooley", in which Medb raises an army to invade the Cooley peninsula and steal the Ulaid's prize bull, Donn Cúailnge, assisted by Fergus mac Róich, a former king of the Ulaid in exile, and opposed only by the teenage Ulaid hero Cú Chulainn. 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, lives, loves and deaths of the characters, and the various conflicts between them.
The stories of the Ulster Cycle are written in Old and Middle Irish, generally 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 and mostly realistic, although supernatural elements intrude from time to time, and heroic exaggeration is common. Cú Chulainn in particular has superhuman fighting skills, the result of his semi-divine ancestry, and when particularly aroused his ríastrad or battle-contortion transforms him into an unrecognisable monster who knows neither friend nor foe. Evident deities like Lug mac Ethlenn, the Morrígan, Óengus and Midir also make occasional appearances.
In contrast to 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, and often involves single combats between champions. The characters' actions are sometimes restricted by religious taboos known as geisa.
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 Mór, who it says was a contemporary of the Roman emperor Augustus (27 BC - AD 14). However, some stories, including the Táin, refer to Cairpre 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.
Some scholars of the 19th and early 20th centuries, such as Eugene O'Curry and Kuno Meyer, believed that the stories and characters of the Ulster Cycle were essentially historical; T. F. O'Rahilly was inclined to believe the stories were entirely mythical and the characters euhemerised gods; and Ernst Windisch thought that the cycle, while largely imaginary, contains little genuine myth.
Elements of the tales are reminiscent of classical descriptions of Celtic societies in Gaul, Galatia and Britain. Warriors fight with swords, spears and shields, and ride in two-horse chariots, driven by skilled charioteers drawn from the lower classes. They take and preserve the heads of slain enemies, and boast of their valour at feasts, with the bravest awarded the curadmír or "champion's portion", the choicest cut of meat. Kings are advised by druids (Old Irish druí, plural druíd), and poets have great power and privilege. These elements led scholars such as Kenneth Hurlstone Jackson to conclude that the stories of the Ulster Cycle preserved authentic Celtic traditions from the Iron Age. Other scholars have challenged that conclusion, stressing similarities with early medieval Irish society and the influence of classical literature, but it is likely that the stories do contain genuinely ancient material.
The Ulster Cycle provided material for Irish writers of the Gaelic revival around the turn of the 20th century. Lady Gregory's Cuchulain of Muirthemne (1902) retold most of the important stories of the cycle,as did Eleanor Hull for younger readers in The Boys' Cuchulain (1904). William Butler Yeats wrote a series of plays - On Baile's Strand (1904), Deirdre (1907), The Green Helmet (1910), At the Hawk's Well (1917), The Only Jealousy of Emer (1919) and The Death of Cuchulain (1939) - and a poem, Cuchulain's Fight with the Sea (1892), based on the legends, and completed the late John Millington Synge's unfinished play Deirdre of the Sorrows (1910), in collaboration with Synge's widow Molly Allgood.
- Compare Táin Bó Cúailnge from the Book of Leinster pp. 164-166 with Diodorus Siculus, Historical Library 5.29, Julius Caesar, Commentarii de bello Gallico 4.33
- Compare The Tidings of Conchobar son of Ness §15 with Diodorus Siculus, Historical Library 5.29
- Compare The Story of Mac Dá Thó's Pig and Bricriu's Feast with Athenaeus, Deipnosophists 4.40, Diodorus Siculus, Historical Library 5.28