What does the subject line "Declined" mean?
What exactly is the problem that you're experiencing?
It's not clear from your post.
Adding money/cash credit to your account doesn't buy any mobile data allowance.
It just tops up your cash credit balance.
What membership plan are you on?
If it's the €12.99 plan then that comes with unlimited Irish national mobile data allowance so you shouldn't need to buy more.
If you're on a legacy cheaper plan with limited mobile data allowance and you've used that up then you'll need to buy a mobile data add-on to continue using mobile data until the end of the membership plan month.
If you need to contact 48 support about this then see the links in my signature below.
