Carolan was best known in his own day as a singer-songwriter. He was an itinerant - he made his living by touring from one place to another, staying in the big houses and entertaining the gentry. Many of his compositions that survive are named for individual patrons, and most of them were composed as songs in the Irish language.
Carolan often used an older tune, to write new words for. An example of this is "Seabhac na hÉirne" ("the Hawk of Ballyshannon") which is a poem to Katherine O'More. It uses the tune "Port Atholl" by Ruaidhri Ó Catháin (c. 1570 - c. 1650).
Because Carolan's tunes were catchier than his quite personal Irish words, they have survived better, and a number of his songs are lost while their tunes survive.