Taranis is one of the last pan-Celtic gods. The etymology of his name also makes him the only clearly Indo-European deity I have come across so far (though Eponis the horse deity is a likely second). Taranis comes from Proto-Celtic Toranis, which means thunder. He resembles Thor, Deor, and Zeus and has both the same root word, Dyeus, and the same basic weapon. Also in common with Norse Thor/Deor and Greek Zeus, he was likely the leader of the Celtic pantheon at some point in their history. As such he would have been a war god.
However, circumstances were different for the Celts which is why the same deity has a different place in their pantheon. The more I research into the Celts’ religion the better I understand why it developed the way it did. The Greeks lived in an enclosed region – isolated from the rest of the world but their various city-states were mutually accessible. That meant that any invading force was able to remain in contact after the conquest. This ease of communication helped them overwhelm the indigenous population’s pantheon. Greek deities are consistently portrayed throughout Greece for this reason.
Rome began as a single city-state which guaranteed that the native religion was consistently portrayed. The Norse constantly interacted with each other from the moment they migrated into the Scandinavian region through raiding as well as trading. Their interchange allowed their gods to maintain both their positions in the pantheon and their personalities throughout Norway, Denmark, and Sweden and down to Germany.
The Celts, however, dwelled in lands that were unevenly invaded; the Irish were not overwhelmed by all the waves of people that Britain was, and Britain was not invaded by as many groups as the continent saw. Britain and Ireland remained in fairly close communication for most of the centuries they were independent (the Irish Sea Province), whereas the English Channel often served as a barrier to the rest of Europe. The results of these varying levels of influence were multi-fold.
It is a sociological fact that an invading culture has to do something with the religion of the conquered. Smothering it by killing its followers only makes the religion a source of rebellion. Instead, the indigenous gods are often incorporated into the religion of the conquerors. In Greece, Zeus’ many love affairs are normally seen as a literary symbol for Indo-European tribes invading and rendering the local goddesses subservient. The existence of previous generations are another means of assimilation; as the son of the previous ruling deity Zeus had a legitimacy that was far more insidious than simple conquest.
The Celts may have attempted a similar approach, but because of the distances involved that approach would have resulted in different priorities, different relationships, different compromises, and in the end a different pantheon of gods throughout the Celtic world. It could not have been otherwise.
Because of this, local beliefs seem to have had varying degrees of success in dominating the new religion. As has been seen the Children of Danaan, a pre-Celtic pantheon, dominated Ireland and several of its gods are known on the continent. Taranis, an Indo-European god, is known throughout the Celtic regions but doesn’t seem to have been dominant anywhere.
The end result is that if a Celt had traveled from Bulgaria to Ireland in the first century before the Common Era, he probably would not have recognized many of the gods that were worshipped along the way, and of those he did recognize many of them would have seemed out of place to him.