What I mean is, if I wanted to RP say a celtic woad raider using a nord character would it work or is it just silly? The reason I ask is that I'm considering trying to RP a celt type warrior for my 3rd playthrough.

Bretons are the closest to Celts. But, that said, Celts are decidedly mixed IRL, including Viking blood from numerous raids and settlements.

True, this is why I'm confused about how to do it, maybe if I stick to ancient Irish celts? My new warrior is called Garaidh after all.

Bretons would be more suitable to Celts, IMO.

But anyway, to answer the question in the title, no I don't think it's silly at all. Many of us make characters that are inspired or influenced by either fictional/non-fictional people or real world cultures.

Personally? I'd stick with Breton then. Nord wouldn't really make much sense. But then, it's going to be kind of hard anyway since there aren't any pikes or spears about. Curious as to how you envision this Celtic warrior. Primary skills?

Not silly at all, you paid for it to do what you like with it if you're looking for an in game similarity the forsworn sitiuation with the nords is very similer to that of the celts with the saxons, not sure if you could get a breten that looks right though.

No, not at all.

Many people use different sorts of inspirations for their character.
One of my characters is based on a certain Subculture. Appearance wise.

I personally think: "I'm a celt in skyrim" is kind of silly. Don't mind if you do, but I do mind the idea.
It's just taking one thing and placing it in another. Now if you get inspired by celtic culture and create a nord or a breton based off what you've heared/learned, that's cool, and you'd create your own thing. That's what roleplaying is about to me and that's what satisfying about it, creating your own character on your own journey.

Not opposed to taking one fictional thing and using it in skyrim. Just more of a fan of things that are made in the world they are in.