Your phone should work there om any membership but you'll need cash credit to pay for outgoing and incoming calls and outgoing texts - calls/texts are not covered by membership/addon allowances when roaming outside of Europe.
Data will not work as it only works when roaming in the EU on a Guido membership.
You seem to be using an iPhone in which case you need to be aware of this issue due to the stpuid way that Apple do things with iPhones.
Ideally make the roaming profile change before your leave Ireland for USA.
If you don't and need to make it there then you may have to try many times before it works.
Toggling airplane mode on and then off and trying the reconfiguration quickly and it may help to catch the local net work long enough to make the reconfiguration change.
What problems did you have in Spain?
Were you on a Guido membership?
Did you get any network connection at all?
If not it could be the "iPhone abroad issue" mentioned above.
If you did and calls/texts worked but data did not then it may have been the APN field of your internet settings changing from 48months.liffeytelecom.com to something else which doesn't work.
In that case you need to edit the APN field, set it back to 48months.liffeytelecom.com, save the change, reboot the phone and see if data starts working.
Hope this helps.