For playing tunes in Irish sessions I rarely need anything other than low D, G and A harps. You can play in first, second, third, fourth and twelfth positions to get all the keys(modes?)used in Irish. The Paddy Richter retune is very important for the G harp especially but no harm can come from having all your 10-hole harps in that tuning. Then you have just the odd tunes with accidentals which can be tricky to get by bending (for example, it's hard to get Cnat on a D harp). You either skip or bluff these occasional tunes or you equip yourself with a couple of chroms, in D and G. Or get a Hohner XB40 if you are into getting missing notes by bending. I've never found much use for Melody Maker and, especially, minor-key harps. In Irish, the "minor" tunes are nearly all either Dorian or Aeolian mode and can therefore be played on standard-tuned harps. I speak not of genres other than Irish.