Actually, I had the Druid/Priest Math trained in and from Ireland living with a small tribe of Gauls called the Boga-Treveri, an offshoot of the Treveri mentioned in Caesar’s Gallic Wars, but my own invention. Druids were international throughout the Celtic world, and traveling between tribes was fairly common, or so I have read. It is said they actually renounced their own tribal allegiances when they became Druids (or Bards). There was a Seeress of the Boga-Treveri that predicted the dominion of Rome over all of the free Celts, and that’s where the quest begins, to try and avert the future, then they realize they cannot stop it, so they have to figure out how to use the ancient magic of the Sidhe to migrate. I don’t have a problem with incorporating the Helvetii into this too, but in my original Celtic campaign they did it all as a race against time to first try and unite the Celtic world against the Romans, which was destined to fail, and then they figured out how to summon the Sidhe to make the magic gates work; ancient stone circles, the most famous of which is Stonehenge. The Sidhe they summoned through the use of their blood sacrifices were, in fact, actually minions of Darkness, but it worked and got them out of there; to the short term advantage of the Darkness.
The actual Sidhe that had built the gates, which greatly predate our modern scientific calculations, used them as something like transporters to observe our earth and to remove samples of creatures to preserve, since the Sidhe (Elves) are kind of hard-core preservationist conservationist environmentalists with an eye towards keeping all living species alive, they don’t believe in the “Law of the Jungle” or “Survival of the Fittest”, there are plenty of empty worlds where, say Wooly Mammoths can live without being hunted to extinction or have their habitat destroyed by climate change. Their world, for instance, has no such dramatic climate change.