The Book of Invasions

Lebor Gabála Érenn (The Book of the Taking of Ireland) is a collection of poems and prose narratives that purports to be a history of Ireland and the Irish from the creation of the world to the Middle Ages. There are a number of versions, the earliest of which was compiled by an anonymous writer in the 11th century. It synthesized narratives that had been developing over the foregoing centuries. The Lebor Gabála tells of Ireland being settled (or 'taken') six times by six groups of people: the people of Cessair, the people of Partholón, the people of Nemed, the Fir Bolg, the Tuatha Dé Danann, and the Milesians. The first four groups are wiped out or forced to abandon the island, the fifth group represent Ireland's pagan gods, while the final group represent the Irish people (the Gaels).

Today, most scholars regard the Lebor Gabála as primarily myth rather than history. It appears to be mostly based on medieval Christian pseudo-histories, but it also incorporates some of Ireland's native pagan mythology. Scholars believe the goal of its writers was to provide an epic history for Ireland that could compare to that of the Israelites or the Romans, and which reconciled native myth with the Christian view of history. It is suggested, for example, that there are six 'takings' to match the "Six Ages of the World".

The Lebor Gabála became one of the most popular and influential works of early Irish literature. It is usually known in English as The Book of Invasions or The Book of Conquests, and in Modern Irish as Leabhar Gabhála Éireann or Leabhar Gabhála na hÉireann.

Purporting to be a history of Ireland and the Irish, the purpose of Lebor Gabála Érenn (hereinafter abbreviated as LGE) was three-fold:

firstly to unite the population by obliterating the memory of previous and different ethnic groups, secondly to weaken the influence of pre-Christian pagan religions by converting their gods into mere mortals, and thirdly to manufacture pedigrees into which the various dynastic groups could conveniently be fitted [2]

...
Wikipedia