The free site www.irishgenealogy.ie has civil death record images of the actual records from 1871-1970. The records for 1864-1870 are currently just index records but we expect that eventually the site will add the images of these records. Death records show the date of death, name of the deceased, townland, cause of death and the name of the informant and in some cases the relationship of the informant to the deceased.
I see five index records fro a Bryan Ryan from 1864-1868. Two of the records are close to where he lived. There is an 1866 record in the Drogheda registration district which I assume is the one in mentioned in your note and an 1864 record in the Thomastown registration district in Co. Kilkenny. This Bryan was 55 when he died.
If your Bryan lived in Gowran civil parish when he died, his record should have been recorded in the Kilkenny registration district.
You can contact the civil authorities and try to get a copy of the 1866 record or wait until the images are loaded online. Here is a link to the site which has instructions on getting a copy of the record. It costs five euros. https://www.citizensinformation.ie/en/birth_family_relationships/registr...