A myth dispelled on the Guiness site:
Is it true that you get a much better pint in Ireland?
For GUINNESS® brewed outside Ireland the water will be different of course because we always use pure, fresh water from local natural sources. In blind tests though, (with a bunch of highly cynical journalists!), none of our sample could tell the difference. That said, all GUINNESS® sold in North America is brewed in Ireland at the historic St .James’s Gate Brewery in Dublin. So, the only real difference you should be able to spot is the pub that you’re drinking in!