First you need to put together some paperworks to prove that you are tax resident in Ireland which I don't think it will be too difficult. Ask the Irish Revenue to send you a official letter that states that you are tax resident in Ireland then I think you would probably need to have a certified company translate the documents (check with the italian embassy for that, I think they have a list of companies that do official translations), I'm not sure if you need that or not though. You need then to go to the office where the pension is drawn, with that document maybe call them or contact them first,
I'm not allowed to post URLs for some strange reasons, however there is a module on the italian Pension Board website (INPS) that once filled out and sent or delivered, should relieve you from the Italian tax according to the treaty between Italy-Ireland, against the double taxation, so you will only pay the tax in Ireland
Search in Google , download and send is the document titled (in english), it is a PDF file look for the INPS website on the search list
" Application form to claim the exemption from the italian pay as you earn as applied to pensions and/or other similar remunerations. [ CI531 - EP-I/1 ingl ] "
Good luck, it will take a while to tackle the italian bureaucracy but it will work out in the end.