The Celts were never culturally united like the Greeks, nor were they close enough so that their best athletes could gather for games every four years during their heyday. This left them unable to standardize their beliefs into a coherent whole. Nor was the religion written down till most of the Irish people had been conquered. As a result, the religion’s original form is hidden; those they conquered blended inconsistently with the conqueror’s religion. Local and pre-Celtic gods became part of the pantheon in some areas that were never known to other Celts. Because of the late recording of the religion pan-Celtic deities are sometimes only known from one or two sites. The poor archealogical record makes discerning between less popular pan-Celtic deities and local gods difficult. What I have discussed so far amounts to the following: Lug, a king of the gods that has qualities more like the multi-talented Hermes. Cernunnos and Belatacudros, who seem like two aspects of the Young and Dying God theme so common in the Mediterranean with Adonis and Osiris. Bran, who had magical powers and whose head was so infused with it that he survived for decades after being severed from his body. I know of nothing in all of mythology that matches that.
Others must be searched for. The Norse have Auddumla, a fertility goddess transformed into a cow that creates life by licking away the ice of the world. The Greeks have Gaea and her mirror self in Rhea who are the life-givers of the gods and the world. The Celts claim no original source. The only hint there is of an origin myth is found in the name of the last second-to-last conquerors of Ireland, Tuatha de Danann or the Descendents of Danu. Danu is mentioned directly in none of the Celtic myths. She was prominent in their rites, however, archealogy has confirmed this. And she was greatly respected. The Danube, Dneister, Dneiper, and Don rivers are likely named after her.
The fact that the Tuatha de Danaan called themselves her descendents suggests that they had no knowledge of the male element in reproduction while they were still migrating. Otherwise they would have been known, as the Ionian invaders of Greece were known, by their common male ancestor. That information alone suggests the Tuatha de Danaan may have come to Britain several thousand years before the Celts who would come to worship them.
Which puts new light on what I have written above. In every other group of people I have run across there has been a religion of the conquerors. In some cases, like the Greek civilization, the religion of the conquered has been retained by relegating their gods to lesser positions in the new pantheon or by adding them in as a previous generation of gods. But not with the Celts. That is an assumption which scholars and casual readers have made in looking at the Celts (an assumption I had made). But their religion is completely unknown before they came to Europe. All evidence of it seems to have been overpowered or inextricably entangled with the native religion. That’s a shame, because in preserving them both we have lost each’s uniqueness and with that the ability to appreciate and study them as independent belief systems.
Cei 
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What do you think of a potential symbolic connection between Bovinda and Auðumbla? Both were related to cows, I’m told. Many feel that Ymir/Aurgelmir represent throwbacks to an earlier pantheon to the Teutonic, hence the themes of replacement deities spread out across the pantheons.
You have to remember that the Indo-Europeans are believed to have been a part of one tribe as recently as 3000 BCE, with a common language but also religion. Odin, the Vanir, they aren’t a part of any other IE religion. It may all be pre IE.