It’s no surprise that Hindu’s ancient religious texts preserve the PIE religion best, so comparing that with Celtic Druidism was natural given the thread topic.
Yet I wonder if Druidism has preserved some PIE religion features better than some other I-E religions. (This mightn’t be surprising: the Greeks borrowed Aphrodite from the Phoenicians; some of the Roman gods were Etruscan, and so on.) Mallory’s old book lists several, almost uncanny similarities between ancient Hinduism and, specifically, Irish Druidism, one of which I mentioned here two years ago:
If you click the link to post-in-thread you will see that I immediately got two responses deprecating the thesis (“sacrificing animals … is common to a lot of religions”). I followed up with more evidence:
… but, unsurprisingly, the equation Epomeduos = Ashvamedha was apparently devoid of merit since no support or retraction followed in that thread.
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Of more direct interest to the thread topic would be similarities between ancient Celtic and Italic religions. AFAICT this does not quickly lead to obvious evidence for Italo-Celtic recency. Deity names cognate between Roman and Welsh or Irish include mostly names common to several I-E branches. A possible exception which one source considers to be a cognate pair (but many/most would reject) is “Roman Mars; Irish Morrighan.”
Interestingly, Morrighan is a Goddess of War, contrasting with the male Mars. Celtic Andrasta/Andarta was also a Celtic Goddess of War (or Strength) the above source considers cognate to Hindu’s male Indra.