I would agree that modern day comparison between Celtic and Hindu cultures do not make much sense. However, what is interesting here, at least in my perspective, is that Celtic and Hindu culture may both be offshoots (but not distant cousins) of a common Vedic or proto-Vedic culture. Also, given the similarities between Ancient Celtic and Hindu/Vedic religious practices, I tend to think that one can reach the same conclusion even after ignoring any musical commonality.
If it was just the Celts, one could even ignore that. However, recent historical findings that ancient Mitanni Kings of Western Asia had Sanskrit sounding names and named Indra, Varuna, etc (current Hindu / Vedic gods) as there own gods also shows some commonality to Vedic or proto-vedic culture. So it becomes difficult to ignore both Celts and Mitanni Kings. Now if you consider the fact that Ancient Iranian culture has much in common with Vedic culture, a pattern starts to emerge leading to the conclusion that Vedic culture was the original culture of the region bounded by Europe, Western Asia, Iran, and India or of people who migrated into these regions. As far as I know this is not a well known or accepted notion yet.
This is enlightening in several ways: 1) it points out, as someone mentioned earlier, that Ancient Celts were not barbarians (probably a myth propagated by Romans in any case), 2) If Ancient Celtic practices were based on Vedic practices, then combined with finding about Mitaani kings of Western Asia and ancient Iranian culture, it leads to conclusion that there was a fairly advanced culture/religion in existence in a Europe/Western Asia/India region before Roman/Islamic conquests (maybe this is already well established), 3) and finally the possibility that some parts of (or all of) ancient Hindu Vedic text of "Rig Veda" may have been composed outside of India (can lead to lot of controversy in Indian/Hindu context).
In any case, I think more historical evidence would be useful to either prove or disprove this.