Why might food spoil faster in Ireland than the US?
I would think added preservatives are the most likely candidate, for example, Azodicarbonamide is frequently added to bread in the US, but banned in Ireland and most of the rest of the EU. Since it has a preservative effect it is a reasonable candidate for a specific causal factor.

But as Keegan points out there are an awful lot of possible factors so it's impossible for us to say with any certainty.