You may get the marriage you are looking for in the civil marriage register as there is a record of 'non Catholic' marriages that goes back further than the 1864 date when registration generally started for BMD's in Ireland.
St Thomas's Church of Ireland church was on Marlborough Street parallel to O Connell Street but it was destroyed by fire in 1922 and nothing remains. The charred records are in the church library on Braemor Road.
The Representative Church Body Library is open Monday to Friday 10 to 1, and 2 to 5
RCB Library, Braemor Road, Churchtown, Dublin 14. You should phone in advance of a visit as it is a small place.
I came accross this bit too
Presbyterian Parish records for the counties of Northern Ireland are kept mainly in P.R.O.N.I., although some records are also available on microfilm in the Presbyterian Historical Society in Belfast. Records for congregations in the Republic of Ireland are usually held locally by the minister.
Note: There is no one finding-aid that records all Presbyterian congregations in Ireland, from the 17th to 21st Centuries. Consequently, it will require research simply to identify whether records for a specific congregation and time-frame survive, and where they can be accessed