Si Bheag Si Mhor (One big one small) refers to two brial mounds. In one of them the remains of an ancient celtic hero (please don´t ask me who - I don´t know, I regret) are suspected. And there was a dispute between two families which one the right one is, because each of these two families owns the ground of just one of these brial mounds. So, because it seems to be a story from long times ago it is high-grade suspicious that original lyrics were in gaelic. (Furthermore I´m not sure, if O´Carolan ever wrote something in english...)