Tuatha Race

[Jasper]

I'm pretty sure they're proper names from Celtic Myth, I believe taking place in what is today Ireland. In the same vein as Firbolg or Fomori, if you're familiar with them. A loose and possibly modern "translation" would be Fey or Faerie. In many game related material about Faeries I've read (e.g. for Ars Magica), Fey Nobles are often called Sidhe, and supposedly descended from the Tuatha de Danaan.

Hero Kings like Nauda of the Silver Arm were of the Tuatha de Danaan. Other main characters include The Dagda, Breas the Beautifull, Balor of the Evil Eye, The Morrigan, etc.

For a brief but entertainingly illustrated read, check out the "Book of Conquests" by Jim Fitzpatrick. Out of print, but the first (of two) volumes is still readily available Last I looked. It's not quite the real stuff, but IIRC he does list his sources.