Ellie,
For records of Non RC marriages from 1st April 1845 onwards, you have some choices. The original certificates were copied. One set is held by GRONI (General Register Office of Northern Ireland). That set is an original (ie with the couples signatures). A duplicate set was created, by Register Office clerks, and that is held by GRO Roscommon. It does not have the bride, groom and witnesses own signatures but should othereise replicate the GRONI certificate. You can view the GRONI set on-line. The Roscommon records are not viewable on-line. Both sets should contain exactly the same information but due to human error with indexing etc, sometimes a record will appear in one set but not the other. However the GRONI set is cheaper and more immediately accessible. And for records after 1922 (partition of Ireland) GRONI has the only set of records for NI.
You can view the original certificates on-line on the GRONI website, using the “search registrations” option:
https://geni.nidirect.gov.uk
You will need to open an account and buy some credits. It costs £2 (sterling) to a view a certificate.
You can order a photocopy of the certificate from GRO Roscommon for €4 (euros). http://www.welfare.ie/en/Pages/Apply-for-Certificates.aspx
You have to download and print off the form. Then either post or fax it back to them. You can’t e-mail it to them. However if you want them to e-mail the cert to back to you, they will do that, so tick the relevant box.
Put the reference details on the form (anywhere). Don’t worry about leaving some boxes blank. As long as GRO have the location, name(s), year, quarter (where there is one), volume and page number they should find it.
In addition, rootsireland and the UHF (subscription/pay to view sites) also have the same records but in transcript form, so you don’t see the original documents. And transcripts obviously carry risks of errors.
Roscommon doesn’t have any church records. Church of Ireland records are held in various locations. Some are still held by the church, some by the RCB library in Dublin, PRONI in Belfast has copies of some and many are lost. Rootsireland has some transcripts on their site. For Newtownards, Church of Ireland, all records pre 1884 were destroyed in the 1922 fire in the Four Courts. Ironically they had been sent there for safe keeping. So marriages post 1845 survive because the civil Registrar kept a duplicate copy from that date onwards, but all the earlier records are lost.