Someone I know, wants to run a campaign in Celtic Britain. He normally uses Pathfinder but is looking for something more appropriate to the background.
closed as not a real question by Brian Ballsun-Stanton Apr 30 '13 at 13:27
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I suspect that the answer depends entirely upon what tone you want to have in your game. If you want something that's truer to mythical source references, then you should look at GURPS Celtic Myth as a sourcebook for either GURPS or some other rules-system (Ken and Jo Walton wrote the book, and Jo Walton knows her some Celtic myth).
If you want "King Arthur and the Celts", then you can use Pendragon along with the supplements Pagan Shore for Ireland and Savage Mountains for Wales -- except of course that King Arthur is post-Roman and that can mean a lot. Still might have some useful resources, but the game as written is most definitely post-Roman.
If you want something less "realistic", then you can look for the RPG (now out of print) that Mongoose published based on the 2000AD comic, Sláine, called Sláine: The Role Playing Game of Celtic Heroes. It's crazy over-the-top, but full of flavour, and has a host of sourcebooks released for it. Unfortunately, it's likely that it'll be hard to track down because it was a licensed property, now I believe out of print, so PDFs are not (afaik) available: you'd have to hit the used market.
There is a free Mythic Ireland setting PDF by John Briquelet (direct link) that contains a lot of useful flavour and cultural detail, assuming you want some mythical and magic touches to your pre-Roman Celts. Even if not, the feeling of the culture – especially the details of honour, the political organisation of the Celts, the role of Druids and the law – is conveyed impressively well and can be used separately from the more mythic beliefs and truths of the setting. It also gives you a solid grounding in what they believed was true, even if it's not actually true in your own Celtic setting.
The PDF happens to be for Savage Worlds, but like most sourcebooks for multi-genre systems, it's valuable regardless of what system you're using.
It seams that there is a Pathfinder sourcebook in developement called Keld: an Iron Age Celtic ENworld: Iron Age Celtic setting ideas...