I live 35 miles from SNN as the 757 flies and the loss of the AA 757 BOS service came as a blow to SNN as loads were always good (can't comment on the yield).
This again was due to deals and alliances (AA/Aer Lingus) and now there are many times when Aer Lingus is full, very expensive or both on SNN-BOS and vv.
SNN - BOS is likely to be profitable May - October. The added cost of a MAN sector compared to extra income is questionable on a 757 but the extra seats on a 767-300 may just make such a route viable but I wouldn't put my last cent on it.