Perhaps a humorous footnote to this thread...I saw an ad in today's NY Times (page 8 in the Style section)for a service called luggagefree.com They will ship your luggage for you (ahead of time) so you don't have to carry it. Minimum charge is $95.

BTW jaspertl, I never mentioned that I think for an Ireland trip where I assume you'll be traveling around making several stops, I agree that 2 smaller bags will make more sense. IMO, 1 big bag works if you're going to 1 destination, unpacking & staying put.