I don’t remember the details, but I have a couple of books on O’Caronlan and indeed if I remember correctly there are comment from his contemporaries stating that he wasn’t the best player.
In fact if I remember correctly it was one of the people who became one of his first patrons who, after hearing him play, said something like, “So… have you ever thought of composing instead?”
He got his patronage for his writing, not his playing. Also keep in mind that Irish harpers had a reputation for being outstanding. To say O’Carolan was not the best is not to say he was horrible. I’m sure he played quite passibly, but just not up to the standards of some of the other well known harpers.
So while he played the harp his fame and the patronage that came with it was for his composing not playing.