Well then why am i getting this for a single fare from LHR to JFK on May 4th 2012?
Because you're looking at one way flights. Firstly, (in relative terms) not many people require a one way flight (certainly not the casual leisure travelers you anticipate can spend a day getting there via Dublin), and secondly, most who do require a one-way flight, and aren't paying on company expenses, know that it's cheaper to buy a return and just not turn up for the return leg, as is the case here.
This flight to BOH isn't to serve the 0.001% of travelers that want to spend over 24 hours getting to New York (and most of those will take the QM2), its to serve the market between Dublin and Dorset. The Ryanair flights used to be popular, but it was another case of the 738 being too much aircraft for such a route. The fares were low and they made very little auxiliary revenue onboard and through baggage sales. The ATR-72 is far better suited for the route.