What an interesting question! I see Martin has already offered some amazing tips to help get you started and I'll be glad to assist you too. :)
To get us started, I also see you mentioned that your Great-Great-Grandfather was in the military. May I ask if he was in the Navy specifically?
If memory serves, there was a specific provision on marriages to Naval officers about that period. In order for Naval Wives to receive a proper pension, their marriage had to be registered with the Navy. Before 1945, registration of marriages across Ireland wasn't mandated, and as an inland city, Fermoy might not have been able to document their marriage well enough. By contrast, Dungarvan was a port city, with a Naval presence. Marrying there may have made it easier for their marriage to be registered so that your Great-Great-Grandmother would receive a pension.
This is just an idea but I do hope that it can help with your search. I would suggest reaching out to the National Archive
for more information about these provisions a well as any documents that they may have on record.
There are also plenty more experts in this community who we're sure would be thrilled to offer a helping hand!
Best of luck,