Dave,

The preservatives that may be added to the stuff would be as a result of various Food and Drug administration policies in countries other than Ireland.
Or it may be that Guinness loses some of the flavour when it is more than 10 days old. I have no proof but I am sure I heard that preservatives were added to the exported stuff.
I know that Cadbury's Chocolate for instance does not taste the same in Canada as it does in England. Canadian authorities insist on certain ingredients to be added to meet their specifications etc. The Canadian Cadbury's is definitely not as smooth and/or creamy as the stuff in the U.K. That is why many ex-pats are willing to pay more for imported Cadbury's rather than the local product.