Yes, I realize that the Celts are real people. And yes, I know that Warlords of Erehwon is a fantasy game. But the former is getting an army in the latter, as the game's designer, Rick, has been adding various real-world peoples into the game. So, why not check out what they're able to do?
From the website:
Rick’s been busy tinkering with Warlords of Erehwon, expanding the game in both historical and fantastical directions – you can find many additional resources on his website but for this article we’re focussing on Celts and Celt Warbands.
Rick: A few folks have asked for a specific list to cover historical Celts. Although it is a fairly simple matter to construct a Celt Warband from the existing Barbarian list, I’ve pulled the relevant entries into one place and made some adjustments to terminology and detail to create a specific list for ‘Celts’.