Mick, I can't understand why these references didn't appear to me when I searched Google Books. However, I've checked via the links, and they are perfectly legitimate. You have my gratitude

It's especially surprising to see "Indian 'Buck' corn" in use in the U.S. - even if the Irish song was made here.

It would now obviously be pedantic and perverse to insist that "Indian buck" in the song refers to anything other than cornmeal, even though the song is at least 60 years earlier than the next earliest example. To say that it does not would seem to be stretching coincidence beyond the breaking point.

Overall, though, it must have been a very rare regional term. It isn't in the multi-volume, seemingly exhaustive Dictionary of American Regional English either.

(For now, I'll stick with my suggestion that "buck" meant "buckwheat." Clearly it was meant figuratively when applied to (coarsely ground) cornmeal, presumably the Native American equivalent of animal fodder fed to people.)