Not sure what exactly you mean by "credit" here but you will need an active membership and cash credit to pay for ougoing and incoming calls and outgoing texts while roaming in North America.
http://www.48months.ie/membership-plans/going-abroad
Note that your membership landline call minutes, mobile call minutes and text allowances will not work there but you still need an active membership to use services.
All outgoing/incoming calls and outgoing texts will be paid for from your cash credit balance.
Internet/data roaming will not work and you will have to use wifi where available for connected apps and internet access.
Hope this helps.
