Not sure where you mean by 'local library' but Coleraine Library, Queen Street has microfilmed copies of some local church records. Box 70 for Macosquin (Camus) Church of Ireland only has Vestry Minute Books 1793-1869.
The church microfilms in the library collection were originally purchased by Coleraine Branch of North of Ireland Family History Society and were housed in the Irish Room collection until it was closed and some of the resources were moved to the main library. The microfilms were purchased from PRONI. If PRONI hadn't gotten permission to film those particular church records then they are not in the PRONI or Coleraine collection.
Coleraine Registry Office are fantastically helpful and if they couldn't locate the marriage in Macosquin C. of I. or anywhere else (their records are now indexed) it sounds as though the marriage either took place outside the area, names mistranscribed or somehow missing from the index.
The marriage records for Macosquin C. of I. would be in local custody but should be the same as the registers in Cloonavin.
If you post the names of the bride and groom or any details that you do have it's very likely someone here will be able to find a reference of when and where the marriage did take place.