I just want to post this link in case it has been missed by those readers interested in Celtic polytheism and Paganism. Alexei Kondratiev (1949 - 2010, of hallowed memory) was one of the most important scholars and early leaders of the Celtic Reconstructionist scene His deep understanding of archeological, folkloric and linguistic aspects of Celtic cultures helped us articulate such things as the Triple Cosmos.
Alexei left behind a number of resources (His book on Celtic ritual needs a reprint.) and one very useful item is the Lorekeeper's Course, kept at Celtic-Nation.org. There probably isn't a better hundred-page summary of what we can safely claim to know about ancient Celtic religion. When you do your own reading in the sources recommended, you might reach a different conclusion here and there, but this is solid, pre-digested and summarized stuff.