The Celts aren't a single race, it's a word used to describe many different nationalities whose languages have the same descent. I've compared a lot of things to the Celts, and am an expert on Celtic history myself, and whilst there is Irish in the Nords and Highland Gaelic in the Ashlanders, there is almost nothing with the Bosmer.
And, as others have said, it's impossible to say the races are based on a single real-life race. The Orcs have Mongol traits, the Nords have Norse traits; but they also have Afghan traits and French traits, respectively. The Ashlanders have many ideas from the Assyrians, the Native Americans, the Israelis, and the Gaels, and the Bretons are almost entirely Breton (I don't know why people didn't get that one almost as soon as the Bretons were introduced to the games), French, and English.