AFAIK, the main reason why RE have kept the ATR42s is because it is somewhat tricky to get the ATR72 into places like Sligo or Donegal (plus there is probably no demand for a 70 seater anyway). Aer Arann would need to sort out if it is worth keeping just one ATR42 for markets such as INV, BLK, CWL or IOM is sensible or not.
I think the main issue is whether or not Aer Arann would keep the ATR42 if they should lose the PSO - and if not, whether or not IOM could sustain an ATR72. The route has 3.500 pax/month, which would give a decent 80% load on a daily ATR72, but only 40% or so with a double-daily flight. Question therefore is if the market does need a day return option or not. If so, anything larger than an ATR42 might be a problem and a Manx2.com 4x daily 19seater service the better solution.
A few aircraft based here and there with a few in Cork and that would be about it!
Their CEO said a couple of months ago that RE is interested in getting ACMi work overseas in order to get a bit less dependent on their scheduled services across the Irish sea.