Flying to Ireland has usually been the cheapest way to get from the USA to Europe for awhile now. With the current sale on Aer Lingus, fall travel from two east coast hubs has been lowered even more. Lowest fares are available for Mon-Thu travel with a $20 surcharge each way for Fri-Sun travel. There is a Saturday night minimum stay requirement and one other noteworthy item in the fine print is that these fares are not eligible for frequent flyer points/mileage accrual. Tickets must be booked by 7:00pm ET on Jul. 11, 2012.
New York to Dublin $563 round-trip including taxes (Travel between Sep. 10 - Oct. 28, 2012)
New York to Shannon $554 round-trip including taxes (Travel between Oct. 5 - Oct. 28, 2012)
Boston to Dublin $723 round-trip including taxes (Travel between Sep. 10 - Dec. 12, 2012)
Boston to Shannon $715 round-trip including taxes (Travel between Sep. 22 - Dec. 11, 2012)
And if you're looking to go elsewhere in Europe, you may be able to find cheap flights from Ireland to the rest of Europe on one of the many discount European airlines. Just make sure you plan plenty of time inbetwen flights when booking two separate flights and diligently read the rules for travel on discount airlines.