I had a look for Loughlin/Laughlin/McLoughlin in the parish of Tamlaght Finlagan that were Church of Ireland in the 1831 census. Sadly there aren’t any. All were Presbyterian.
I also looked for Dooris (and variants). I couldn’t find any at all. The name does exist in the county (as Dooris, Doorish, & Dooras) but none living in Tamlaght Finlagan in 1831. All the Dooris entries in the 1831 census were RC, so this looks to have been a mixed marriage.
I searched the statutory marriage registers from April 1845 (when they start) to 1853 but did not see a marriage for Robert & Margaret. You might want to search the pre 1845 records for Tamlaght Finlagan. The marriage records start in 1796. I don’t know if they are on-line anywhere but if not, there’s a copy in PRONI in Belfast, if you can get there.
Possibly DNA testing may be a way of matching with others who have additional information about where the family originate. Family Tree DNA reportedly has more people with Ulster roots than any other company. That obviously increases the chances of finding a match. You might want to try them or, if you have already tested, you can transfer your results to them for no fee.
The North of Ireland Family History Society is running an Ulster DNA project in conjunction with FTDNA and can offer testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).
