Having flown with both Aer Lingus and Ryanair to Dublin they're both pretty good on the short hop. When I heard that Aer Lingus were leaving GLA for Aer Arrann, I had a look at the prices as I had never flown on at ATR72. I say I LOOKED at the prices as they'd shot up from the A320 previously on the route.
Slow turboprop with high cost base versus B737-800 with possibly the industry's lowest cost base. Unless this is a serious attempt to feed the long haul at Dublin, I don't see what Aer Arrann are bringing to the table. People are happy with the lack of amenities on such a short flight.