Civil Registration of non-catholic marriages started in Ireland, north and south, on 1 April 1845. The commencement date for registration of births, deaths and catholic marriages was 1 January 1864. A full NI birth, marriage or death certificate cost £12 from GRONI, but there are a number of cheaper options available.
The best option is to visit GRONI in Chichester Street, Belfast and pay £12 for a full day’s “index search” which must be booked several weeks in advance. This entitles you unrestricted access to computerised indexes and to 4 “look ups” of the actual registers. Any additional look ups are then charged at £3 each. These provide all the details recorded and included on the full price certificate. I would recommend doing your pre 1921 birth and death searches on the LDS website first and taking a list of references to check with you. At GRONI the marriage and post 1922 indexes are excellent and very easily searched, while the earlier birth and death entries – although computerised – are less convenient.
You can order a transcript of the details on a marriage certificate by phone from the Registrar’s Office at the relevant District Council for £3 or £4 plus postage if you know the names of the bride and groom and the exact date and place of marriage (Belfast City Council don't provide this service). These details are available from the Emeraldancestors.com online subscription database and look ups are often offered on this site by people with subscriptions.
You can search for all NI registration certificate references 1845-1921 online at the LDS Pilot Site
http://pilot.familysearch.org/recordsearch/start.html#p=collectionDetails;t=searchable;c=1408347A search on this site will give you the name, registration district, year, quarter (if any), volume and page number taken from a microfilm of GRONI’s own index. This information is sufficient to allow you to order a certificate from GRONI for £6 (DO NOT TELL THEM you got the above details from the LDS website or they will charge you £12!!). Certificates normally arrive at UK addresses within 1-2 weeks.
http://www.groni.gov.uk/index/order_certificates.htmCounty Antrim and Down certificate details (including Belfast) may be available online from AncestryIreland.com for £4 which is a slightly cheaper option, but doesn't provide the official document.
http://www.ancestryireland.com/database.php?filename=db_QUISPre 1921 certificates for all parts of Ireland are also available from the Republic of Ireland GRO in Co Roscommon. They offer a cut price “research certificate” which is uncertified and is an actual photocopy of the original registration entry. This costs €6 plus postage (or €4 plus postage with the full reference from the LDS site) and can only be paid for in cash or by sending you credit card details in the post, which many people consider to be a security risk. Also the service can take around 4 weeks. GROI also provide a public search room in Lower Abbey Street, Dublin, but the indexes are quite antiquated and it’s a good idea to do your searching on the LDS site prior to visiting.
http://www.groireland.ie/apply_for_a_cert.htm