Lebor Gabala Erenn
The Book of the Takings of Ireland
better known as
The Book of Invasions
This text--found in both the Book of Leinster and the Book of Fermoy--is the core text of the mythological cycle in Irish literature, as well as the earliest known history written by the Irish. It tells of the successive invasions of Ireland by different tribes, from the creation of the world to the coming of the Milesians (Iberian Celts):
§01-13: Biblical History
§14-25: Gaedhil History
§26-29: Cessairians
§30-38: Partholonians
§39-47: Nemedians
§48-54: Fir Bolgs
§55-64: Tuatha De Dannan
§56-60: The First Battle of Magh Turedh
§65-: The Milesians
Of the above invasions, the Partholonians and the Nemedians are also discussed in the Historia Brittonum, which is generally dated to the 8th or 9th century, if no earlier; Giraldus Cambrensis also mentions these two tribes.
In truth, Ireland was settled by several groups of people: nomadic hunters and gatherers; pre-Celts and the Cruithan (Picts); Iron-age Celts, first from Northern Europe, and the second possibly from Spain. This--a hypothesis--is reflected by the existence of the Milesians. Also, there is the curious story of the "Black Irish"--that the existence of the short, dark-haired and occasionally olive-skinned people, usually found in the west of Ireland, are the result of the intermarriage of shipwrecked Armada sailors and local girls. Some say this is history; others say it is myth. Either way, it isn't hard to see a connection between the pre-historic Milesians--son of Mil of Spain--and the Spanish sailors of the 16th century.
The translation provided here is the Irish Texts Society edition, with the final section--that of the Milesians--provided by the Cross & Slover translation. To my knowledge, they are both public domain.