You can get a very cheap pre-paid ("pay as you go") phone in Ireland at any phone shop. If you can, forward a US landline to that number before you leave (you won't be able to forward your AT&T iPhone number at a reasonable rate). This may be possible from your office PBX?

I've used Flowroute (www.flowroute.com) in the past for this - got a US number from them, and set it to forward all incoming calls to a specified Irish mobile number at a reasonable rate - less than 15c / min, I believe.

For outbound calls, you may be able to get an add-on of some sort from the Irish mobile provider (O2 / Vodafone / Three / Meteor) to make these cost effective. Alternatively, if you will have access to WiFi, just use Skype / Google Voice.