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What do you mean 'the american appstore appeared' ? Are you in the US ? You need to be in a country to use its store, so if you aren't in the US then you can't use the US store. If you are in the US then you can try contacting iTunes Support and ask them if they can remove the balance from your account so that you can change countries : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

If you have Euros on your account then presumably you have your Irish address on it ? If you do then try going to the bottom of the Featured tab in the App Store app on your iPod and tap on your account id, tap on 'View Apple ID' on the popup and log into your account, and then select the Country/Region section and select Ireland.