You can't.
You need an active monthly membership plan for outgoing services, including mobile data, to work.
You can buy data add-ons but they only work when you also have an active membership plan.
You would normally only buy a data add-on if you're on a legacy plan with a limited mobile data allowance which you've used up before the end of the membership month.
That allows you to continue using mobile data until the end of the membership month.
This is moot on the €12.99 plan which comes with an unlimited Irish national mobile data allowance.
Mobile data add-ons can also be purchased in order to get more UK/Europe roaming data allowance if you've used up the normal plan roaming data allowance.
If you need to contact 48 support about this then see the links in my signature below.
