The Well Below the Valley, to which I referred in the list of links I mentioned earlier, is not a Cruel Mother (Child #20) variant but a traditional form of The Maid and the Palmer (Child #21); they do have motifs in common, of course.  It's been recorded from tradition only once in Ireland, from John Reilly, though obviously it must have had some currency before then. The two texts in Child are from England and Scotland.

I don't know of any Cruel Mother variants (Irish or otherwise) involving incest, though a number of other songs deal with it.