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.
There are a few questions here, so let me address them individually:
Guinness Draft, the "standard" Guinness is not always brewed in Ireland, so that may be one reason for a different taste in Ireland than in US. As gannolly commented, the age of the beer also affects the taste, as well as the cleanliness of the beerline (if the beer is on tap).
Guinness has a few beers, one of their biggest beers is a Foreign Extra Stout. This is brewed in various counties. This is a higher ABV, roastier beer. I think it has double the ABV of a "standard" Guinness.
One other important fact is emotions. If I am sitting in the Guinness Brewery, looking out over Dublin, drinking a fresh, perfectly poured Guinness, I am in a very happy place. This happiness will affect my overall experience of the beer. If you take that same keg, keep it cool and fly it overnight to the US and taste it there, it will still be good, but it will not be as good, because you are missing the experience. This said, the beer can also be just as good, because you remember the feeling of drinking that beer in Dublin.
Here are links to the various Guinness Breweries and the beers that are brewed there. http://www.ratebeer.com/brewers/st-jamess-gate-diageo/13/ http://www.ratebeer.com/brewers/guinness-cameroon/13088/ http://www.ratebeer.com/brewers/guinness-nigeria/3194/ http://www.ratebeer.com/brewers/guinness-ghana/5070/
In foreign countries, a particular brand of beer is often produced under a licence to a local brewer.
The accuracy with which these local brewers reproduce the original beer varies greatly.
For example, Australians noticed a change in the local Guinness when the local licensee changed from CUB to Lion recently: http://www.news.com.au/national/south-australia/lion-brews-the-first-guinness-in-south-australia-in-40-years/story-fndo4dzn-1226508692750