Elves are not Celts. Also, Celts are only viewed as a race of tree hugging hippy type culture by those who only concern themselves with popular history/archaeology. In fact Celts are much less naturalistic than most think, its is only in their art that any kind of naturalism shows. Tolkien, as a very well educated scholar would have been well aware of what Celts were really like.

With regard to invasions and loss of empire, I think its unwise to look at any of the invasions of Britain as models for the decline of the Elves. Modern archaeological interpretations of all invasions of Britain end with life carrying on very much as it always had done for the majority of peope, only the top levels of aristocracy end up changing.

I would see the decline of the Elves much more as the decline of the Roman Empire, spreading out and covering most of the known world, then slowly declining through both internal and external factors, until they all end up back where they started from.

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Anar kaluva tielyanna!

Mornië alantië

Nemo Propheta acceptus in patria sua.

Dulce et decorum est pro Tolkien mori.

"Yes, he can speak eight languages, and he has nothing of interest to say in any of them" - Unknown Radio4 book reviewer on an unknown language professor.

"Besides you need people of intelligence on this, er... quest... mission...er, thing!" - Pippin.

"Well, that rules YOU out Pip!!!" - Merry.