The best option of all would have been to have kept the 737-500s - after all, if it has been good enough for Lufthansa to have hung on to their -300s and -500s it might have been a good idea too for Aer Lingus especially for Cork and Shannon operations to mainland Europe.
The ERJ190/5 series are bound to be far too expensive and the A318 sub type is far too heavy a frame in the same way a 737-600 would be.