Interesting!
At one stage in the 1960s it was accepted that at least a third of the population in Australia had at least some Irish ancestry (I'm one of the ones that missed out, explaining my difficulty with Irish decoration of tunes) but I'd never heard the term "black Irish" until I visited South Carolina in 1991. The fact that I'd not heard it is no evidence that it wasn't used, of course, but I suspect I'd have heard it if it had had reasonable currency.

When I did hear the term, in SC, it was in the context of a discussion about the differences between slaves (we'd been to Charleston and seen the remnants of the slave marketplace there) and the indentured labourers from Ireland. The locals were telling me that a fit and healthy slave had much the same capital value as a D9 dozer and, while there was a lot of abuse of such people, it was in the owners' interests to maintain the value of their capital investment. There was no such "interest" in maintaining the health and fitness of indentured labourers, used (during the period being discussed) as the major labour force to drain the mosquito-infested swamps of the Carolinas.

The indentured labourers in these swamps were almost all Irish (according to my informants) and died in extraordinarily high numbers and the term "black Irish" took on a different meaning, not necessarily "accurate" in this thread, but still evocative.

Cheers, Rowan