I found the marriage on the LDS site. What you have to do is note the place, year, quarter (if given), volume and page number, and then search for a matching entry for the spouse. There are several ways of doing that, either by searching with the full name (which you evidently have) or searching on forename only, or by using the search facility on this other site, where you can search by year, quarter, location and page number (but no name):
http://search.ancestry.co.uk/search/db.aspx?dbid=2572The reference to the volume & page number is the Volume and page number of the Register that the marriage was recorded in, by the Registrar. It’s therefore a bit surprising that the person you spoke to at Armagh Registry office was apparently unfamiliar with that, but there you go. If you can quote the place, year, (quarter if one is given), volume and page number when ordering certificates you generally get a reduced fee (as it takes them straight to the correct entry). You can order a copy of the certificate from GRONI in Belfast but a cheaper option, in my opinion, is to order a photocopy from GRO Roscommon. Provided you quote the place, year, Volume & page number clearly on the application form, they generally only charge €4 for a photocopy. It takes about 7 to 10 days from when they receive the order. You have to fax or post your order in., You cannot order a photocopy on line. However you can download the application form from their website.
www.groireland.ie/ Or if GRONI already have the enquiry in hand, I'd give them the location details referred to above, as there may be a reduced fee.