Cernunnos is Gaulish, which is a different geographical region entirely. Where Gaelic Celtic culture encompasses (to the best of my knowledge) Ireland, Scotland, and some of Britain, Gaul consists (/consisted) of mostly continental countries, e.g. modern day France, Switzerland, etc., east to Germany. (-Ish. It’s more of a diaspora, kinda complicated.)
Cernunnos is found in the continental Celtic countries of Gaul. He is not actually nearly as widespread as professed to be by most sources, since most sources have a strong New Age influence re: Wicca.
I’d recommend reading Serith’s paper on this, since it goes over the geography as well as actually analyzing the god himself, and perhaps looking up articles on JSTOR.