and may I suggest you request a photocopy of the marriage cert of John and Anne from the State records in Co. Roscommon. Fee of 4 euro. www.groireland.ie
and in particular download the Application form for Marriage / Civil Partnership ( http://www.groireland.ie/apply_for_a_cert.htm
It is simple and of course you dont have PPS numbers etc but you can guess the marriage year range and the people in Roscommon are very helpful and get stuff out within a week usually. The Cert would normally have fathers name, address so that would get you going.