In Julian May's Saga of Pliocene Exile, the Tuatha Dé Danann and Fir Bolg are recast as warring aliens from another galaxy inhabiting prehistoric Earth, the "Tanu" and "Firvulag", respectively.
As for the third, Luserke's attempt to stir Anne McCaffrey's (Dragonriders of Pern series) interest in his project was a failure, so he tried Julian May (Saga of Pliocene Exile).
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