Actually, most of Celtic migrations were long before that, petering out about 100 BC or so, around the same time as Roman Republic was becoming the Roman Empire. As a whole, they ended up in the Brittish Isles, France (Gauls), a good chunk of Spain and parts of Germany. Oh, and northern Italy (Cisalpine Gaul).
How related all the groups really were could be debated, but archaeologically they shared the same material culture, and Romans considered them all roughly part of the same culture. Of course, most of the Celtic influences went away on the continent long before the period of stories that Yeates was drawing from. (Thank you, Romans, Goths, Vandals, Allemani, Burgundians, Lombards... and other rude neighbors.)
So now that I've bored everyone, I thought I'd say the most impressive thing about the Buttery Spirit is that he's still reading as blue, instead of the transparency just turning him gray, or the same color as the background.