Agnes ROBINSON death

I am looking for the death registration of my grandmother Agnes Robinson.  According to one of my cousins, she died on 11 April 1957 in Larne, Northern Ireland.  Agnes Robinson (née Thompson) was Presbyterian, married Samuel Robinson on 1 January 1902 in Banbridge, Down, and had links with Gardenmore Church.  I have searched on ancestry.com Ireland Civil Registrations and www.irishgenealogy.ie and familysearch.org.

I found the birth registrations for all her children on irishgenealogy.ie but cannot find their marriage registrations.  Where else should I be looking for these?

I would be very grateful if someone could make some suggestions.

Thanks, George