Ok, such a quick study can't do the subject justice, of course, but here's a start:
What We Don't Know About the Ancient Celts __ "When one says that one wants to study and, perhaps, reconstruct the religion of the ancient Celts, it is well to be clear about whom one speaks. Celtic describes a language group which, over time, has divided into two strains -- P-Celtic (Brythonic) spoken in Wales, Cornwall and Brittany and Q-Celtic (Gaelic) spoken today in Ireland, Scotland and the Isle of Man. Celtic speaking people inhabited much of Europe for millennia and their descendants live on today. Each locale and each time offer different views or hints about the nature of the relationship of the Celtic people to divinity." Learn what tools are available for this study and how different Celtic societies approached the divine. - By Rowan Fairgrove - http://www.conjure.com/whocelts.html
Now the Dutch....