I understand you're referring to their civil marriage certificate?
I don't understand how you could have been told you need to know the district where they were married.
The General Register Office in Ireland had all the certs up to and incuding 1922 before Northern Ireland was established. The application for the marriage cert doesn't ask for that information. Go to the http://www.groireland.ie
website and click on "apply for cert". Then click on Marriages, Certificate Application Form in English.
You should be able to order the cert with the information you already have.