Not to mention that we really know very little about what religion the original population of Prydein and Eire followed. Yes, it's assumed to be something similar to Druidism, but most of that legend about there having been some sort of "true and pure" Druidism practiced there, without any taints from Roman, Greek, Asian and Mesopotamian religions is probably just a myth created by the neo-Tribalists and Prydeinic Nationalists.
For all we know, they may have had completely different gods and rituals. Just look at the differences that range across all of Odinism, or Wuodanism, Wotenism, and Mercurism depending on what part of Europe you're in. Sure, the names of their gods are similar, but many of their myths are very different. The Eastern European religion of Tiewism, among the Goths, is actually the same group of gods, with a different Chief. And that's just in a culture that diverged from a similar base in the last two thousand years, add in another thousand years or so of divergence among the Celtic peoples, and who knows what you'll get.