Just got this from Amazon Customer Care. It'll be fine it seems.
Hello,
Yes, you can purchase the Kindle with Special offer from retail store in U.S and you can use the Kindle in Ireland without any problem.
However, you won't be eligible to claim the special offers.
We're always looking for new ways to help readers discover Kindle. Selling Kindle in physical stores is a natural extension of that philosophy.
We're selling the same Kindle and Kindle DX available on Amazon.com in retail stores. However, retailers determine their selection and pricing. Please contact the retailer directly if you have questions about Kindle pricing and availability.
For locations, see our Help page:
www.amazon.com/help/kindle/retail
You can also learn more about all available Kindle models on our product detail pages:
Kindle Wi-Fi:
www.amazon.com/kindlewifi
Kindle with Special Offers:
www.amazon.com/kindlespecialoffers
Kindle Free 3G + Wi-Fi:
www.amazon.com/kindle3G
Kindle DX:
www.amazon.com/kindleDX
Thanks for your interest in Kindle.