According to the Book of Invasions (Leabhar Gabala) the Partholonians arrived in Ireland from the eastern Mediterranean, possibly Greece, 312 years after the Flood. Prior to their peaceful invasion the land was occupied by another race of people led by Lady Cesair and her father Bith, who was supposedly Noah's grandson. They had hoped to escape the Flood but were unsuccessful - except for one mythical survivor known as Fintan (Bith's grandson) who somehow managed to stay afloat. Fintan seemed immortal and he lived on for many years at Dun Tulcha in southwest Kerry. Incredibly (although not that unusual in Irish myth) the ageless Fintan appears again some thousands of years later, in the sixth century CE, to settle a dispute at Tara regarding the limits of the royal demesne with his testimony of remarkable memories.

The Leabhar Gabala was written in the 12th century by monks who were nobly attempting to organize various fragmented mythical traditions into one body of literature. Consequently the book contains references to the Old and New Testaments of the Bible mixed with oral legends told through the ages by bards and seannachies, including roles played by local pre-Christian gods and goddesses. It is flavored by the monk's religiosity, smatterings of political propaganda, family pride, and a healthy dose of nostalgia for earlier times, written with the belief that current events could be explained or justified by knowlege of the past.

The original race of Lady Cesair and Bith was destroyed by the flood. Then Partholon, also known as Barthlomaeus, came and cleared the land to cultivate crops and breed animals. Under the Partholonians, rivers and lakes were created. There was the first natural death and burial in Ireland. Just as the first incident of adultery was about to occur, the immortal Fomorians - sea pirate monsters who each had only one leg and one arm - engaged the Partholonians in battle near what is now Dublin Bay. The Fomorians were defeated but would return later to engage in warfare with later settlers.

The Book of Invasions obviously contains much symbolic mythology whose true meaning is probably lost in the mists of time. But it also gives specific geographical locations and concrete historical events. In early Celtic literature, time is elastic and non-linear. Events often do not seem to occur in the straight line sequence that we are accustomed to today. The life span of Fintan is a prime example. The Partholonians are said to have lived in Ireland for 5000 years, then "between two Sundays a great mortality struck them and they all died" except for Tuan MacCarill who proclaims "but there is no disaster without a survivor to tell the story. I am that man..."