It is said they landed from afar on Beltaine. They cleaned up Erie of the Fomors, which I assume were also alien. It is said they were defeated by the Milesians.

I don't theenk so. I'm sure they had superior technology and cleaned up Erie FOR the Milesians, and went away, sorta. Went 'underground' at least into a parallel dimension. I think they come and go, not really having that much interest in the majority of the mortal flotsam garbage that walks the Earth, though they may associate with limited numbers who will work with them on this or that.

I'm under the impression the Tuatha (Pronounced TOO-ha. Gaelic and English use the same letters, minus the accent marks with English, but they're not pronounced the same.) and Fomors are ancient enemies from worlds away, and there was a Tuatha agenda to destroy every last one of them. Also, is there any record of Fomor skeletons in Ireland? Me thinks the Tuatha used some sort of vapouriser to get rid of the Fomorians.

Speculation of course, but it seems to make sense.