O'Carolan is generally considered to have been a far better composer than player. His compositions are mostly thought to be in the "renaissance style(s)", in contrast to "traditional" styles. Regardless of such scholarly interpretation, right or wrong, most of we mere mortals consider his work to be trad and beautiful. I have a copy of Grainne Yeats' book on O'Carolan and I'll check it later today, just to be sure I haven't strayed too far here.

As to a "technically correct" answer to the question of the thread, "trad" is anything in the public domain.