In his notes to a reprint of John & William Neals's " A Collection of the most Celebrated Irish Tunes", Nicholas Carolan (a contemporary historian) states that the melody of the song "Cuckoo" (or possibly "Bonnie Cuckoo") is the origin of O'Carolan's "Sheebeg and Sheemore", which was written around 1691.

"Sheebeg and Sheemore" is considered to be O'Carolan's first composition; he apparently based his melody partly on the older song. Because the melody is so instrumental in character, I would think O'Carolan's version wouldn't lend itself to being sung. It's generally thought that O'Carolan didn't actually sing most of his 'songs' - even those melodies he to which he had written lyrics. Apparently he is supposed to have recited (or chanted) the lyrics rhythmically while playing the tunes.

Although O'Carolan is most famous for writing beautiful melodies, he did in fact write several poems - including an elegy to his wife.

As stated, there's a wonderful book by Donal O'Sullivan on O'Carolan's life and music. I understand that it's recently back in print. It has most of his tunes and the stories behind how many of them came to be written.