That aside, one minor (pun intended) point: every diatonic harmonica is
by definition a modal instrument, since it only contains the notes of a
single diatonic scale (overblowing and bending aside), and therefore
anything played on it, in any key, is played using a mode of that
scale. So it's really a question of what mode you're playing on what
instrument. You mention using a C harp played in A minor for Carolan's
Welcome; this is an Aeolian mode (unless you're bending or overblowing
notes on the harp for this piece, which I don't know and can't say,
since I don't know the piece).
....So there are a few different ways to do the job. With Irish music, it
might come down to which position on which harp will let the player
execute various kinds of trills, grace notes, and other ornaments most
easily. I can't say which of the various choices that might be. But if
it's working already, I agree that there's no point in looking
elsewhere.