I search the IFHF's online database at ROOTSIRELAND.IE. It has abt 20 M church, civil, census, etc records for MOST Irish counties, including Tyrone.
For a William Graham and Elizabeth Oliver marriage in 1879, Co. Tyrone, with the parents' names unknown, I found 1 record. This is likely (but not for certain) for your ancestors. A view of the detailed record will likely show their parents' names and other details. NOTE - Protestant records are the least likely to have survived.