From a scheduling point of view, operating a flight that was purely based on long-haul feeder traffic would be a bit of a pain in the but. You would want to arrive into Dublin in the 7:15-7:45 range and depart about 10:15-10:45.
That's too long to leave an aircraft hanging round on the ground, but not quite long enough to squeeze in another rotation. Even at the long end of that, by the time you've factored in 3 half hour turnarounds, you get about 2 hours of flying time. Not many viable destinations in that range from Dublin in an ATR (both in terms of how far the ATR can fly and the fact that you'd be up against 737s and A320s on LPL and MAN). Maybe the Isle of Man or Blackpool could work, but that's about it.
So you end up having to swap aicraft, with all the crew-rostering hassle that's involved there.