But you fail to forget about the night in Dublin (costly!) and flying with the hangover the day after...
I would of thought anyone who is going to connect on to an Aer Lingus US Destination (Boston, Chicago, New York/JFK or Orlando) via Dublin would stay at an Airport Hotel. The Clarion at Dublin Airport offers rooms from €69/£57 per night, I myself think that's very reasonable.
Flight Option's to New York for One Adult single on May 4th 2012
Flight Option 1 / Aer Lingus:
BOH to DUB (Plus Fare) | £47.99
Clarion Hotel (One Night) | £57.00
DUB to JFK (Economy) | £378.00
Total: £482.99
Flight Option 2 / British Airways:
LHR to JFK (Economy) | £892
Flight Option 3 / Virgin Atlantic:
LHR to EWR (Economy) | £754
Other Advantage via Dublin, US Preclearance. The Choice is clear, and going via Dublin gives you a night in Dublin City with over £400 extra in your Pocket.