Now you're getting into very rocky ground that has been the subject of much speculation, but that does not mean that there is not room for further speculation and discussion.

I'll throw a few mixed things in here re Druid Cults.

We know for a fact that this religion existed. Claudius saw them as the single biggest threat to his empire, hence why he wiped them out.

The long white dress is not very typical for Euro/Celt peoples. It's more like a desert dwellers garb.

The Celtic Religions (very different to the Celtic people classification) have more parallels of their sacred items to be found in the middle east/Asia.

Celtic Knot work is very similar to Islamic artwork.

There are strong blood/language links between the Moroccan Burba people and the Basques - Europes only true Celtic people remaining.

There were strong trading links between the Phonecians and Ireland and the Ancient Greeks also traded this way.

There is no archaeological evidence that supports any invasion theory either in Ireland or GB. Missionary work (combined with good pyrotechnics?) are a more likely route for entry.

It is just possible that they were outcast priests/alchemists who knew the secret of Greek Fire .. but that's stretching it a little bit I think.