I know I'm compounding thread drift, but I just throw this in on the Guiness debate.

At the time when the Park Royal brewery was still in operation I met one of their brewers socially and asked him about the supposed difference between the Dublin and London brewed products.

He said that they were indeed deliberately being made slightly differently because the English were not patient enough to wait for a pint to be pulled in the Irish way and the English version was made to settle more quickly.

Whether they now make a different version in Dublin specially to be shipped over here I don't know.