Just as far as Carolan goes- Irish trad music happens to be something that I know quite a bit about, and I'd have to say that Carolan's 18th century harp music would be an innacurate analogue for 3rd century BC celtic music, as the wire-strung harp is an essentially modern instrument, dating from no earlier than the 11th or 12th century AD. Carolan also composed in the modern diatonic mode (heavily influenced by older irish modes of course) so his music still sounds "modern" and is still popular today. SO it's unlikely that Vercingetorix would have been listening to Planxty Irwin.