On the question of the "Celtic"-ness of the Irish, there's an interesting
book by Simon James, "The Atlantic Celts," Univ. of Wisconsin Press, 1999.
James is a serious scholar, a British Museum archeologist currently (1999)
at Durham University, so he can be taken at face value, whether you agree
or not, it's something to consider.
Of course the Annals of the Four Masters paint a very interesting
geneology for the Irish, and once again, you pays your money and takes your
chances. Much more fun to contemplate than "serious" inquiry, and a lot