My mate Mile Lydyat and I play a lot of Carolan tunes and as it's not in the 'Book' by O'Sullivan he contacted Bonny Shaljean the harpist who said it wasn't by Carolan. In searching further we found that is actually from a song Tho' Leixlip is Proud, about Leixlip near Dublin having lovely woods etc . It seems that O'Neill may have seen Tho' (Though) as Thos. short for Thomas and bagged it as another Carolan tune. It's certainly a great tune but then there were lots of Baroque influenced tunes and songs from operas etc at the time from all over Britain and Ireland. Carolan was certainly open to all influences. I'll put up some links