Last year, DUB had the 7th highest number of weekly flights to North America in Europe, just behind FCO. LHR is comfortably #1, followed by (not necessarily in correct order), AMS,FRA,CDG and IIRC #5 was LGW. I was very surprised that DUB was so much busier trans atlantic than the likes of CPH, MAN, ZRH, MUC, ORY etc. apologies for not having the exact figures owen, I read it in one of the British papers during the summer.
Keep in mind though, that's movements, not seats. DUB may well overtake FCO this year in movements, but a larger number of FCO movements will be widebodies, so I suspect it'll be comfortably ahead of DUB in terms of capacity