It’s true, and it has been empirically evaluated: Guinness really does taste better in Ireland.
The results of the Guinness-tasting t-test were clear. Pints consumed in Ireland had a mean GOES score of 74, compared with a score of 57 in pubs outside Ireland. While Ireland may not necessarily keep the best stuff to itself, the science is clear: Guinness tastes better over here.
Being a Man of Science myself, of course, I’m not going to simply accept this claim, but will have to engage in some spot-testing and verification next week in Dublin.