When roaming in the US you connect to a local network there and need cash credit on your account to pay for outgoing calls (€1.99 per minute), incoming calls (€1.89 per minute) and outgoing texts (39c per SMS). Incoming texts are free.
http://www.48months.ie/membership-plans/going-abroad
http://community.48months.ie/t5/Most-Popular-Questions/Going-abroad/m-p/35043/highlight/true#M60
You also need an active membership but the membership allowances don't cover any services while abroad.
Internet access over carrier data will not work at all while roaming abroad so you are restricted to wifi where available.
DO NOT buy the EU roaming calls/texts add-ons as they will not work outside the EU and you will be wasting your money.
If you have an iPhone then be aware of this issue when roaming abroad:
http://community.48months.ie/t5/Need-Help/Iphone-6-no-network-when-overseas/m-p/32727#M11081
You may need to make that change before you go to avoid hassle trying to do it while abroad.
Getting iPhones working while roaming abroad seems to cause problems for some people.
Another option while abroad - depending on hoe much you use your phone, how many and what types of calls etc. you make/receive etc. - is to see about getting a SIM abroad for use for the duration of your stay. In some cases this might be more cost effective but obviously you won't be able to use your "normal" Irish mobile number if you do this.
Hope this helps.
