The refrain "Hèman Dubh" is actually just vocables, à la hi ho ro 's na horo eile. Sometimes the vocables match actual Gaelic words, and get spelled accordingly (dubh, eile), but they don't mean a damn thing (unless you buy the theory that Gaelic vocables are remnants of Pictish verse, in which case they mean the same thing that the unheard falling tree in the forest sounds like).
Thanks. It was my first suspicion that all of this might be just vocables, nothing more.
The spellings of vocables aren't entirely fixed but they do tend to follow certain patterns - "Mantú" doesn't look right, though there's nothing specifically wrong with it that I can see (and "Hèman" violates the "leathan ri leathan agus caol ri caol" rule). I can't speak to spellings anns a' Ghaeilge (hint, hint).
Right - "Mantú" is
rather strange-ish, even to this armchair enthusiast nonspeaker (and thanks for the hint, BTW
). And something was bothering me about "Hèman" but I couldn't put my finger on it. Oddly enough, usually when I see a violation of the broad-for-broad, slender-for-slender rule in Irish (and I've noticed it doesn't really apply in Old and Middle, and is violated orthographically only on rare occasion in Modern; "Gaelach", for example), the lapse stands out like a sore thumb for me, I've become so habituated to the standard. So, normally Scots Gaelic shouldn't hit me any differently in that regard.
FWIW, the arguably problematic spelling "Hèman Dubh
" goes at least as far back as 1949. [Edited throughout to correct for a missed fáda. Pesky things. ]