Shamrock350, it depends how you look at it. In relation to 174 seat 757 being higher cost. If the tickets cab be sold on a sun route at a fair price nothing to say the 757 in this configuration couldnt work. The is a opportunity cost here in terms of say getting a 320 in to Shannon for such a route and hence other revenue potentially bring eroded ex Dublin. That is assuming there are no surplus 320s, won't be the case for long...
However, what's to stop the utilisation of the 757 being increased? Why not turn it around and fly at different times? Would mean that the flight wouldnt be at the same time every day?