Is Guinness really different in Ireland, compared to what's sold in the US? I've never liked the taste of Guinness, on tap or from the can. But I have heard that it's much better tasting in Ireland, and the export one is completely different. Is there any truth to this, and if so, what are the differences?
Guinness can taste slightly different and have a different head based on where it's coming from. A big factor in this is how long the beer you're getting has been in the keg. People in Ireland drink Guinness a lot more than in the states, so kegs are replaced more frequently i.e. fresher beer. This is why if you go to a bar in Ireland and not a lot of people are drinking Guinness, you shouldn't get one. 

Another possible reason you've heard this is that most people visit Dublin when they're in Ireland and have a pint at the Guinness store house. They ensure all the beer there is top of the line and it really is much better there than any other place I've had it.