While Wicca borrows somewhat from the Druidic heritage, don't be misled to believe Druidism spawned Wicca. Crystals, candles, and Tarot have naught but a passing relevance to Druidic practices. But don't rule out Stone Henge, the Druids DID practice there, they just didn't build those stones. Shadowrun makes an admirable attempt at creating Druids that resemble the beliefs of the ancient Celts, but they can't manage to shake that Pythagorean elementalism. Still, it's a lot closer to the mark.
Rifts takes some liberties, but it's also obvious they did their homework.
One product, the Celts Campaign Sourcebook, is definitely a cut above the rest, and I recommend it highly. Conway, D. J., Celtic Magic. Daniel, Sir J., The Philosophy of Ancient Britain. Kennikat Press, Port Washington, 1970. Davis, G., Celts Campaign Sourcebook.