Seven and three quarter hours FLIGHT TIME would be about average, but it can vary quite a bit depending on the strength of headwinds over the ocean.
The Aer Lingus flight is scheduled at eight and a half hours, but this is 'chock to chock' time so it includes taxiing...usually about 15 minutes extra is built in for that.
You generally get there early.
John, just checked BA for same dates I'm flying & it was £700+ ! searched for the best available dates (these don't suit me anyway & got £254 (E373) & you still have to get a flight to Heathrow. I think I'll stick with Aer Fungus !
Another advantage of going to the US direct from Dublin is that you go through immigration in Dublin. In the US then you just need to collect your bags and go through customs.
To get back to the original question ... North Atlantic flight times depend a lot on winds - exact routes are changed daily to minimize headwinds/maximise tailwinds. Air Traffic control restrictions would have an effect also but winds more of an effect.
Genearlly Westbound flights from Ireland would take at least 1 hour longer than Eastbound flights returning to Ireland.
Dublin - NYC flights would be probably 6-6h30 mins flying time ... and the flights in the other direction would be 5-5h30mins (though I think I've been on one that did it in 4h45mins or something)
For Boston you could subtract around 30mins from those times.
The only thing I would add to the via LHR discussion above ....
For a short trip it's probably not worth it from a time point of view (crap and all as Aer Lingus are now) .. also with cheaper Aer lingus fares (and continental matching them) it's further not worth it ... going via LHR (or AMS, CDG etc.) is probably only worth it if you are going to a place that doesn't have direct services from Dublin.
One thing that does bug me about the Aer Lingus flights though is the departure time from the US of many of them .... the "traditional" Aer Lingus JFK-DUB flight departs at around 1830 from JFK ... and gets into Dublin at around 0530 (which is only 0030 NY Time) .. so you get on the plane early evening when you aren't tired... by the time you have your (meagre) dinner etc. then you're already half way to Dublin ... and probalby still not sleepy as it's only like 2100 NY time or something ... then before you know it you arrive in Dublin and it's supposed to be the next day though you probalby haven't had any sleep so you're screwed from a jet lag perspective.
From a Jet lag point of view it's far preferable to get a flight departing the US late (like 2200 or something) ... then you get on the plane .. are tired and have a chance to sleep ... also it means you dont arrive in Dublin at an ungodly hour of the morning.