Pay as you go cell phones in Ireland?

The phones sold in Germany should also work in Ireland if the phones are unlocked. Quad band phones are needed only if you want a phone that works in Europe and the US.

Vodaphone is in Ireland, too. I think you could just top up your minutes like you always do - buy a minutes card. They may get used up a bit quicker because you will be roaming with your German number, but it should work. Call Vodaphone and ask.

My Vodophone cell didn't work in Italy so I assumed it wouldn't work in Ireland. I guess I need to do a little more digging ...

Yes, you can buy a cheap pre-paid phone anywhere in the European Union. Pre-paid phones are often country-locked, and when they are not, they're expensive for overseas roaming. This is not a technical limitation, it's a decision by the carrier. They're often trying to prevent fraud and/or pre-paid billing problems. If you are going to do this often, or visit several countries, the best way to do this is to buy a cheap unlocked GSM phone on ebay, and then just buy pre-paid SIM cards in each country.

I'd agree that a prepaid phone from one country may not be usable in another country without taking some additional step. This summer, we successfully used two T-Mobile U.K. prepaid accounts in Germany. We simply had to buy some type of Euro booster package that sold a bundle of minutes or a fixed amount of data. That protected T-Mobile U.K. from a customer running up the bill because the company immediately took the cost of the booster package out of our prepaid balance. When the package ran out of minutes/data, we got a text asking if we wanted to buy more. Buying more simply required sending a text message to the carrier.