It's not clear to me how you're trying to add credit. Typically, when you have a prepaid plan, you get a SIM card from that plan, which is associated with your account, and you manage the minutes or data on your account through your carrier's website (or possibly their app). Can you confirm that the SIM card in your phone is from Eir? Or are you trying to use some random phone with a random SIM card on Eir?
Oh, and I saw the thread title of your previous post before belodion merged it. Do you really expect anyone to help you after you trash-talk them? And yet here I am trying to help. Something must be wrong with me.