Hi Wes, you are unlikely to find the marriage as the only records available are church records at that remove, it appears even though you found the baptism the marriage was too early, the Penal Laws meant that RC records were not always kept or got lost when a church was subsequently built. I see the birth in Darver parish as you mention and I see a sibling in 1791 on Roots so there is a Patrick and a Thomas.
The more common spelling of Murta nowadays is Murtagh and you could also try Fitzsimons in searches on this site, you get hits on both spellings http://jbhall.clahs.ie/
There is a book on the area called Kilsaran Union of Parishes by a priest first published in 1906 or so, you can find it on Google books and it has grave inscriptions.
On the site above there is a piece on the Wild Goose Lodge murders which occurred nearby circa 1815 I think and there is a Netflix movie of the same name filmed locally, also book on it in case the history interests you.
There are a number of queries on Darver on this site if you want to search the site, it is a common place to receive queries for. Did you try the Flax Growers list of 1796 which may show a townland were they lived. the families were probably tenants, you can find the areas here but if you had a townland it would narrow it down. https://www.townlands.ie/
There is a Facebook page for Louth Genealogy and you apply to join, a chance you could find some actually living in Darver, I live about 25 miles away. There are other pages on Dundalk and around the area. The surnames are still around the place.
Feel free to ask any thing else that comes to mind.
Good Luck
Pat