The marriage was in a Registry Office (common in the case of mixed marriages). Irish marriage certs do not say where the couple were born. However one of the couple should have been resident in the Enniskillen area, for the marriage to have taken place there. If you get a copy of the certificate it should contain their townlands (addresses) which may enable you to trace back further.
Sam’s information suggests Ann at least may have been born there. Graham is a very common name in Fermanagh. They are generally supposed to be descended from Border Reivers sent there in bulk at the time of the Plantation. Consequently a lot of border names are common in Fermanagh eg Armstrong, Carruthers, Nixon etc.