Two points come to mind. Firstly Carolan wrote songs rather than tunes. The lyrics to all but one of these were in Irish, not English. Donal O'Sullivans book contains the one English lyric he found. Travelling from one patron to the next he would occupy himself with composing a new song for his next host. Whilst he may have started by re-using existing melodies I think it's mreasonable to assume that the majority of the tunes he used were original.

Secondly, although as a blind man he could not have written his stuff down, his big patrons were the Gentry in the 'Big Houses' and they could write. These were educated people with educated children. Part of their education would have been musical and it should not be a surprise to find that some of his songs were written down the children or grandchildren of the people for whom they were written.