The Ulster Cycle, formerly the Red Branch Cycle, is a large body of prose and verse centering around the traditional heroes of the Ulaid in what is now eastern Ulster. This is one of the four major cycles of Irish Mythology, along with the Mythological Cycle, the Fenian Cycle and the Historical Cycle

The cycle centres around the reign of Conchobar mac Nessa, who is said to have been king of Ulster around the time of Christ. He ruled from Emain Macha (Navan Fort near Armagh), and had a fierce rivalry with queen Medb and king Ailill of Connacht and their ally, Fergus mac Róich, former king of Ulster. The foremost hero of the cycle is Conchobar's nephew Cúchulainn

The society depicted in the tales of the Ulster Cycle is substantially that of the pre-Christian Iron Age, although filtered through the perspective of its medieval Christian redactors, and contains many parallels with the society of the Celts of Europe as described by ical writers. Warriors fight from chariots, take their opponents' heads as trophies, contend for precedence at feasts, are advised by druids, and fight in single combat at fords. Poets have great power and privilege and wealth is reckoned in cattle.

The cycle consists of about eighty stories, the centrepiece of the which is Táin Bó Cúailnge, or the Cattle Raid of Cooley, in which Medb invades Ulster at the head of a huge army to steal the Brown Bull of Cooley, and only Cúchulainn stands in her way.