What country are you roaming in?
When you buy add-ons the money is taken there and then from your credit balance or card as you pay up front for membership and add-ons.
Edit: I'm just wondering if you are, in fact, still in Ireland and bought the roaming add-ons thinking that you could sent texts to EU numbers from Ireland using them? If so then that is not the case - they are only for sending texts when roaming abroad in the EU away from Ireland. If you managed to send 4 EU texts then they would have cost you at least 25c per text (more if they were very long and used more than a single SMS each) and that might explain why your cash credit balance has reduced? It would also explain why you can't sent another EU text - because you only have 20c and you need at least 25c to send one.
In that case you should still have all 70 EU roaming texts left because none should would have been used up if you are not abroad. You can check this by dialing *100*6# to get your current EU roaming text balance.
Handy short-codes
Hope this helps.
