For most of a millennium this was simply the history of Ireland. It underlies the Annals of the Four Masters in the seventeenth century, Geoffrey Keating's influential history of the same period, and the standard account taught in Ireland well into the modern era. The Milesian genealogies were used by ruling families to establish descent, which gave the text a practical political function and a strong incentive for its continual extension.
It is not history. There is no archaeological trace of successive invasions on this pattern, the chronology is constructed backwards from biblical dates, several of the peoples appear to be learned inventions or projections of tribal names, and the whole scheme is a medieval synthesis rather than a memory. Nineteenth-century attempts to read the takings as a record of real migrations, and twentieth-century attempts to match the Fir Bolg or the Tuatha De Danann to archaeological cultures, have not survived scrutiny.
What it does preserve is real, and it is of two kinds. First, in euhemerised form, the names and attributes of gods: Lugh, the Dagda, Nuada, Brigid, Ogma, the Morrigan and Eriu appear as human kings, warriors and queens with fathers and death-dates, and the same figures are worshipped or invoked as divine in other Irish sources and are attested in continental inscription. The compilers evidently knew they were handling gods and chose to demote them rather than delete them.
Second, and more securely, it is direct evidence for the intellectual world of eleventh-century Ireland, what its scholars read, how they reconciled native tradition with Christian universal history, and what kind of past they thought their country ought to have. As a document of medieval Irish self-understanding it is first-rate. As an account of what happened in prehistoric Ireland it is worthless, and it is worth saying both things plainly.