Hi possum_pie
Depending on which of these apply to your marriage (records held by the GRO in Rosscommon):
3. Registers of all Marriages registered in the whole of Ireland from 1st April 1845 , to 31st December 1863 , except those celebrated by the Roman Catholic clergy.
4. Registers of all Marriages registered in the whole of Ireland from 1st January, 1864 , to 31st December, 1921 , and in Ireland (excluding Northern Ireland ) from that date.
The LDS have the Irish BMD indexes on film.
If you can find the entry and therefore the district, vol, page details, etc. you can then order it via
http://www.groireland.ie/Bren