Some years ago, I visited the Guiness brewery in Dublin, and enjoyed the local brew. I did notice that Guiness tasted better in Ireland than it does here. I also noticed that it seemed to be possible to drink a lot more without feeling the effects.
I had the opportunity to meet and chat with Pete Slosberg, of Pete's Wicked Ale fame. I asked him about this, and he said that the formula is different. The version that we (normally) get here is higher in alcohol content, for one thing. US law prescribes a certain percentage of alcohol in beer. They have to boost the alcohol and the hops for export to the US.
(Hops is a preservative in beer.)
I hope to return to Ireland some day for more testing!