Two short additions: first, there's a reference to "Skibbereen" in the introduction to Hughes, "Irish Country Songs" (1909), so that's the first decade of the 20th century (with the likelihoood that the song itself is a bit older than that). Secondly, in O'Sullivan's "Songs of the Irish" there's a good deal of historical information on both "Eamonn a chnuic" and "Roisin Dubh", together with some other "Patriotic Songs" including "The Races of Ballyhooley". I think O'Sullivan makes some suggestions, with regard to the last of these, that humorously/satirically naming an ignominious retreat/rout a "race" became commonplace or traditional, as in "The Races of Castlebar" in 1798.

I see these two additions have become three. Incidentally, with regard to songs written long after the events, while one version of "The Croppy Boy" is certainly nineteenth-century ("Good men and true..."), isn't the other ("It was early, early in the Spring...") generally regarded as coeval with the events?