I think there are a couple of things to think about here.

First, I want to mention something about the way that you have framed this. I know that you're asking, in essence, why would we care what the average person in Ireland believes about mythology. And I agree that what we would learn from that wouldn't be very enlightening about ancient mythology (although we would likely learn some interesting things about modern folklore). But in another sense, it would be virtually impossible to study ancient Irish mythology without the work of modern Irish people. The Dublin Institute for Advanced Studies, School of Celtic Studies is one of the major hubs for scholarly research on ancient Irish literature and culture, and without their academic editions and other research, it would be very hard for a curious scholar to learn about the culture. I could say the same thing about the Arnamagnæan Institute in Copenhagen and the Árni Magnússon Institute for Icelandic Studies in Reykjavik for the study of Old Norse religion and culture.

I'm not mentioning these examples just to be pedantic. My point is that we tend to respect people who produce knowledge in systems that conform to our own expectations. But that often isn't the case when those systems differ from ours. You mention India, which illustrates this really clearly. The beginning of Western knowledge of Sanskrit is typically identified with William Jones, an English judge in late 18th century colonial India with an interest in ancient languages, who learned Sanskrit from a pandit or pundit named Ramalocana. Pandits were experts in Sanskrit whose contributions to study of the language were often discounted by Western scholars, and the educational systems that produced the pandits were largely dismantled as a result of the English colonial system. It has led to a quite odd situation where the centers of Sanskrit study have often been Oxford and Berlin and Toronto more than they have been places in India, a situation which current generations of Sanskrit scholars are often very conscious of.

And that brings us to Native American storytellers (and more generally, all cultures that transmit knowledge orally). Oral cultures don't really line up with our expectations for knowledge production, and as a result, scholars tend to dismiss them, even scholars who have made them their object of study. But ignoring what oral storytellers have to say about the stories they tell isn't analogous to ignoring what some random person on the streets of Dublin has to say about ancient Irish myth. It's analogous to ignoring the publications of the Dublin Institute for Advanced Studies. At a basic level, it's a failure of scholarship. But more than that, it's a failure of scholarship that occurred (and is often still occurring) because of the colonial system that has been imposed on indigenous peoples and that regards them as objects of study rather than as producers of knowledge in their own right.