Did you have the global plan on the device before you went to Ireland?
Having access to phone calls on your mobile device while traveling internationally is very important. I'm sory you were not able to use your phone in Ireland Mike48. When you returned to United States were you advised as to why your phone did not work. Did you have data roaming turned on the iPhone? In the future is all the information you will need to use your phone internationally http://bit.ly/xZd3pS . Please let make know how I can help.
Your iPhone 5 should have worked in Ireland. It's GSM compatible. Did you have roaming turned on? Did you have international dialing turned on (VZW feature)?