Its been quite some time since I've seen round-trip airfares to Europe for less than $600. That's the rate Aer Lingus is currently floating, and as per the new DOT regulations, those prices include taxes, so there shouldn't be any ugly surprises.
In order to get these cheap tickets, you'll have to book quickly. When the ghosts and goblins of Halloween disappear, so do these fares. They must be booked before Nov 1, for winter travel. As per usual, the lowest rates are for mid-week travel (Mondays-Thursdays) and from the New York City area. But fares from Boston aren't much higher, starting at $649. From Chicago, the starting rate is a still-decent $699.