In terms of environments that would most resemble those where Celtic and Gaelic tribes lived... I'd say probably in the north, probably the areas east of the Rift, and/or around the Rift itself. You could also get away with setting up in the Rainlands, but because the Salisheds have influenced the cultures of the natives, you would have to come up with a reason for why they have kept their culture intact. Since the Salisheds seem to be centered around the eastern Rainlands, the western Rainlands would be best. I imagine the Rainlands would also resemble western (or more likely southern) Europe, so if you want to be near other players, it's probably the best choice.