More great research, Mick.

But I do doubt that the phrase originated in the song, if for no other reason than the song seems not to have been very well known.

Further conjecture is probably not useful, but it may be that "Indian buck" was once far better known than it has been since the 1860s. While plenty of rural Irish words and phrases are recorded in earlier literature, there's no reason to assume that every last one appeared in print, or has been discovered if it did.

About all we can say is that "Indian buck" almost certainly meant "cornmeal" in Ireland in the 1860s, as it has more recently, but that one could easily have lived a long and productive life any time over the past 150 years without ever encountering it.

In other words, it is and was very rare.

The dialect dictionaries are filled with such terms. Neologisms don't always spread far..

The extreme case of "Boontling" in California is not entirely comparable, but you may find it diverting:

https://en.wikipedia.org/wiki/Boontling