As inconsistent as it is, the Irish mythology is far and away the most complete Celtic Mythology that has survived. In that respect it can give you a good feel for what the religion was like as a culture. For instance, the Greeks euhemerized the two historical invasions of their region into two succeeding generations of gods, with the the first two being depicted as evil (Uranos and Chronos both swallow their children) and therefore the emergence of a new generation is made out to be an improvement. With the Greeks the first generation is totally impersonal, the second is full of nature personifications, and in the third the gods interact greatly with humans.
The Celts present their history without that seeming bias. The Fomorians are never associated with evil. For them the only differentiation seems to be that the Fomori had magic, and the Milesians were smarter. The Fomori may lead you into their realm, but they generally try to help mankind.
With that backdrop in place, I would like to focus on the Welsh material a little more. I would like to start with two of the British gods, both near and dear to me from my graduate days. They are often found associated with each other in native archeological digs. Belatacudros was a god of death, closely associated with the fertility deity Cernunnos. Belatacudros was likely associated with crows, the scavengers of the dead. Cernunnos’ symbol was the horns of a stag (his initial syllable of “cer” is probably from Celtic “horn”. Pictures of Cernunnos normally include a cauldron or some sort of vat. Together the two gods made for a sort of two-aspect deity along the same lines as Adonis among the Phoenicians or Osiris among the Egyptians. As Belatacudros he was nature dying and when transformed into Cernunnos he was the rebirth.
Belatacudros was worshipped among the troops along Hadrian’s Wall, probably because he was linked to crows and death. The Romano-British who followed would also take an interest in him. Although they knew of more powerful gods like Lug and Manawydan, they generally chose him, in his shortened version of Beli, as the founder for their dynasties. I can only assume he was so appealing because of that same connection with crows and death. Both are often associated with warrior-kings in the British literature.
Belatacudros also shows up in Arthurian romances (which is where I found him). There he is consistently linked to the holy grail in the persons of Pelles (the Grail King) or Pellinore (one of the protectors of the grail). On the surface that seems odd. As a Christian symbol, the association of death is wrong.
However, now that we know the grail was a cauldron used in the fertility rites by a subsection of British culture, it is feasible to see Belatacudros as an integral component of those rites. In modern parlance, Belatacudros was the Old Year and Cernunnos the New Year. That would explain their contrasting symbolism and undeniable association in the archeological record.
However, their connection with the grail lore and each other would make both Belatacudros and Cernunnos pre-Celtic deities that the Celts had forgotten were foreign gods. I think that’s very interesting. Hera and Aphrodite are two examples of pre-Greek deities. Hera’s function is the cuckolded wife and Aphrodite is a nymphomaniac. But for the Celts, there was no conflict in blending the old gods with the new pantheon and giving them a place of real importance.