The mythological cycle, the oldest cycle, is preserved in the Lebor Gabala. The cycle describes the early settlement of Ireland through a succession of invasions by five supernatural races. The most important race was the Tuatha De Danann, or People of the Goddess Danu. The Tuatha De Danann was the fourth of the five invading races. They defeated two other races, the Firbolgs and the Fomoirans, and were in turn defeated by the Sons of Mil, also called the Milesians. The Tuatha De Danann were the source of most of the divinities that the Irish people worshipped before they became Christians in the A.D. 400's.

The Ulster cycle centres on the court of King Conchobar at Ulster, probably before the time of Christ. The stories deal with the adventures of Cuchulainn, a great Irish hero who can also be considered a demigod. In some ways, he resembled the Greek hero Achilles. But unlike Achilles and other Greek heroes, Cuchulainn had many supernatural powers. He was also a magician and poet.

Many stories about Cuchulainn appear in the Ulster cycle. Probably the best known is the Tain Bo Cuailgne, the Cooley Cattle Raid. In this story, Queen Maeve of Connacht ordered a raid on Ulster to capture a famous brown bull. Cuchulainn single handedly fought off the invaders until the queen's forces finally captured the bull. However, the Ulster warriors led by King Conchobar came to Cuchulainn's aid and drove the invaders out of the country. Queen Maeve plotted revenge against Cuchulainn and later used supernatural means to cause his death.