It can sometimes be more difficult to find 'recent' information like this. The online index to civil registrations (free) only goes up to 1921 for Northern Ireland and I believe Emerald Ancestors (pay site) marriages go up to 1926. Your 2 choices are probably to go do a search in Belfast or check if your local LDS library has the marriage indexes for that period on microfilm to you can get the details needed to order the certificate unless you know where the marriage took place and can contact that place directly.
