The local registry offices used to keep copies of the marriage registers but about a year ago they had to be sent to GRO (some sort of indexing/extraction project?) but you don't know exactly where the marriage took place so that's not an option.
There are several sites that do provide information (for a fee) but there indexes can be used to narrow down the possibilities as to date, etc.
Emerald Ancestors has marriages for N.I. counties up to 1922 and I couldn't find a match for Samuel and Violet.
Ancestry Ireland says they have "Virtually all civil marriage records for Counties Antrim and Down" which doesn't give any indication of how recent the records might be. I found marriages for 3 different Violet Thompsons but none matched with a Samuel Lynn/Lynd.