That was a few thousand years before Celts were around.
Edit: It was also pretty slow; it was a couple of hundred miles across and took three thousand years to disappear, so it's on the order of a few miles in a lifetime.
It sounds like the character has the same total level as the rest of the party, but can only use features from half of those levels, so it's going to be underpowered to the point of unplayability.
I have to ask what those reasons were.