Morgan Llywelyn, the Celtic author whose childhood was divided between the U.S. and Ireland, has written many books on Celtic folklore. I had never heard of her until you Mudcatters posted messages about her books.

Her latest book just came out, January 2016.
"Only The Stones Survive" focuses on the arrival of the continental Milesians, and their encounters with the Iverni, the Fir Bolg, and the Tuatha de Danaan. The decision is made, in the timeline of this book, by the De Danaan to become the Sidhe, and Llywelyn shows them making this decision during a council meeting before a battle with the Milesians.

The speculation that goes into this account will attract some readers and repel others. Llywelyn includes a bibliography at the end for her more speculative source material.

So this is Llywelyn's personal re-working of the material from the old legends preserved in Gaelic manuscripts. Some of this story she has touched on in previous books, especially "Bard" which has more about the sons of the Mil in continental Europe. There have been hints earlier still, in Llywelyn's books "Druids" and "The Greener Shore."

Ireland itself becomes a character in the story, which is typical of Llywelyn with her sense of atmosphere and detail. Some memorable moments feature the Hill of Tara and the Stone of Destiny.