Erin,
You haven’t said what denomination the couple were, but in any event, none of the churches in Aghanloo has any records for the 1700s. The earliest is Magilligan Presbyterian which start in 1812. Aghanloo Church of Ireland lost its early records in the 1922 fire in Dublin and has nothing before 1904. The earliest RC records (Drumachose) start in 1855 plus Magilligan RC in 1863. (There's no chapel in Aghanloo and so it's RC parishioners would have attended chruch in either Magilligan or Drumachose). None of the churches in Limavady has any records for the 1700s either. So there are almost certainly no church records covering the period you are interested in, in the Aghanloo area.
Linton is a surname suggesting Presbyterian origins (ie they’ll be descended from Scots who settled in the area in 1600s) but O’Brien suggests a native Irish background which in turn points to being Roman Catholic. If it was a mixed marriage they almost certainly married in the Church of Ireland as that was really the only church that would marry them at that time. So their ancestors might be buried in any denominations graveyard. (Many graveyards were mixed anyway).
Only the fairly wealthy could afford gravestones in the 1700s and so you won't find many. Most people were buried in unmarked graves. Some have become illegible or broken with the passage of time, but your main obstacle is that 90% of the population simply didn’t have a gravestone.
I looked in the 1831 census for Aghanloo. There were no Linton households listed at all. There was 1 O’Brien. It was a male plus 5 females, all Roman Catholic. Head of household was Alex. They lived in Dowling townland. That’s probably Dowland today. Dowland is on the Aghanloo Rd just north of Limavady. There were no O’Briens listed there in Griffiths Valuation of 1858. In the 1901 census there were 9 houses there. Total population 33. No O’Briens.
Researching in Ireland in the early and mid 1700s is always difficult and often impossible simply due to the absence of records. Unless either family was a big landowner or notorious in some way, there are probably no records of their existence in Ireland to find.
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 and can offer FTDNA testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).