I don't believe I recall ever having been to an Irish session where there was someone there playing melodica, or bass melodica for that matter. I have, however, seen such interesting things as dijeriedoo, washtub bass, jug, chinese hammered dulcimer (terrific player, too!), harmonicas galore, various & sundry odd drums, recorders, piccolos, fifes, and a few other not exactly traditional musical instruments.    Ever think about the washtub bass? It's tons of fun, and much cheaper!