Mike, I checked one way fares for the summer 2009. I found that Aer Lingus has the best one way rates. You have to change planes in Dublin, but even so, the rates are well worth it. I had priced Dulles to Dublin and then Dublin to Barcelona as well as Dulles/Dublin/London. If I remember correctly, a one way fare was $550 or so.
Someone pointed out that although Aer Lingus is a good airline, you need to be aware of small hidden charges that can add up, such as extra charges for bags, charges for booking seats, etc. So, just be aware.
AND, if anyone is interested - changing the topic slightly - I noticed that Aer Lingus has a very nice round trip price Dulles to Dublin in April

(Spring Break for many of us). It was awefully tempting, although I'm abstaining to save money for the 2010 trip!!!!
Jodie