hi! Not entirely sure whether you're just wondering about Druids specifically v. Celts in general, and the British Isles v. anywhere, so I've gone with 'Druids anywhere' and rounded up previous posts that may be of interest

• What do we actually know about ancient Druidism in the British isles?

• Historians, what concrete information is there about the druids? (Big question, I'm sorry)

• So were "the druids" a thing?

• Were there any people in Germanic tribal society that served as religious intermediaries (e.g., shaman, priests, druids, etc.)?

• What was the role of Druids in Pagan Irish religion?

• How shamanistic was the religion/belief system of Northern Europe before exposure to Christianity?

• What is the latest Druids could have existed?

• A girl I know gets indigant every St. Patricks Day, because she believes that St. Patrick conquered Ireland and forcibly converted them, committing genocide against the Pagan druids who lived there. Is this claim accurate? If so, to what extent?

Also, some more info on Stonehenge

• What language would have been spoken during the building of Stonehenge?

• Did the Picts Build Stonehendge?

NB: the historical record is scanty; it might be worth x-posting to /r/AskAnthropology to see if there's additional archaeological/anthropological input

Question about the druids:

Friend of mine says they took their name from oaks, which in Greek is Drys (Δρυς), but it still sounds kind of iffy to me.

Any info on that?