It depends ... I've had it at the brewery in Dublin, and here in the US.
It can taste almost exactly the same, and it can taste very different.
The thing is, the recipe for Guinness does differ, depending on which Guinness you're referring to.
There regular Guinness or draught Guinness (which in the US we can get in the special foaming 16oz cans).
There Guinness Extra Stout that comes in bottles, which we can get in the US in bottles. This doesn't taste like draught Guinness. But I noticed in Ireland that draught Guinness and Guinness Extra have the exact same alcohol: 4.2%ABV. So I'm not sure what is really what the extra refers to. Maybe they make it with a higher percentage of roast barley in the grain bill.
Finally, there is Guinness Foreign Extra Stout. This differs, depending on where in the world it is made.
The stuff made in Dublin is 8.0%ABV. But I never found it anywhere in Ireland. I found it in a shop in London.
This is true ...
Guinness Foreign Extra Stout made in Nigiria is 7.5%ABV. The rumor I heard was that the Nigirian brewery makes it with some sorghum (to cut costs, or to reduce the amount of malted barley required).
Guinness Foreign Extra Stout is made for the people in tropical climates, (in hot weather, the more alcohol, the longer the beer stays "good"). In London, you have to hunt around for it, and look for it in stores in areas where there are a lot of immigrants from tropical countries.
JW