The English death indexes give Eric’s date of birth as 6th Jan 1934. Eric’s death is registered in Portsmouth in August 1998. I don’t see a matching birth in England so he may well have been born in Ireland.
If you think he was born in Derry (or anywhere in Northern Ireland), you could order a copy of his birth certificate from GRONI in Belfast. Contact details for ringing or e-mailing GRONI:
If they married in Northern Ireland, you can ask GRONI to search for the marriage certificate. Obviously if Theresa was born in Northern Ireland they should have her birth certificate too. If she was born in the Republic of Ireland then you need to contact the GRO there.
You can order a research copy (photocopy) of the civil certificate from GRO Roscommon for €4 (euros). https://www.welfare.ie/en/Pages/Apply-for-Certificates.aspx