Well, there was one released in 2001 which has been called at times Vercingtroix, Druids and The Gaul. It stars Christopher Lambert and Max von Sydow and was directed by Jacques Dorfman,
The Wicker Man would probably be the best known one (the original, they did do a remake but I haven’t seen it so can’t comment, sure it still sticks to the central story though which is druid/celtic through and through)
Merlin and Morgana Le Fey of King Arthur legend practiced different aspects of druidism. So most movies about Camelot and the Knights of the Round Table are druid themed to some degree.
@WestRiverrat, the first film that came to mind when I read this question was Avalon. There are some really hot sex magick scenes in that one, I seem to remember…