We flew Continental from Newark to Shannon, and from Dublin to Newark on the return trip. It was a smaller plane - 757, I believe - but it was fine. There seemed to be no problem arranging to arrive at one airport and depart from another. I don't know whether rental car companies would go along with that, though.
Can't say it would be a horrible thing to drive across Ireland twice, if you had to use the same airport for arrival and departure. Those narrow roads sure don't thrill me. I'm glad I wasn't driving. I've sworn I will never drive in Europe.

-Joe-