Cearas is right. The Lebor Gabála Érenn (The Book of Invasions) was compiled by an anonymous Irish scholar in the 11th century from a collection of poems and prose narratives that recount the mythological history of Ireland.

There are people who believe it's a true history, just like there are people who believe in the so-called "Celtic Tree Calendar" that nineteenth century antiquarian Edward Davies concocted from a 16th century history of Ireland called Ogygia by Ruairí Ó Flaitheartaigh. Robert Graves continued this myth and devised the dating system. They seemed to believe it's a secret druidic ogham, which is totally laughable considering druids didn't leave any written evidence of such a calendrical system.

Yes, a calendar was found that may be of Celtic origin: the Coligny Calendar, a series of bronze tablets dating to the first century BC, but it's not written in ogham script and makes no mention of trees.

I guess what I'm trying to say here is that those who do not belong to a reconstructed, neo-pagan faith that has these beliefs as part of its doctrine are better off sticking to what we can know about the Celts and avoiding all the wild notions that Edward Davies, Iolo Morgannwg and others cooked up during the "Druidic Revival" of the late 18th/early 19th centuries. Many documents were forged during this time. It has taken scholars of Celtic Studies years of dedication to undo the damage. No academic today worth his or her salt promotes such balderdash.

There has also been a concerted effort to study the medieval pseodo histories in a more scholarly light. While works such as The Book of Invasions are not considered forgeries, they are recognised as being a tangled web of myth, legend, folklore, Christian hagiography, history and political embellishment. In other words, there's no reliable evidence of an Egyptian princess named Scota having existed.

Edited to add that I'm not saying there's anything wrong with speculation, discussion, religious beliefs, or even wild conjecture, provided we can distinguish the difference between each of these and academic studies, and also to thank William for raising yet another interesting topic.