According to A History of Congregations in the Presbyterian Church in Ireland 1610-1982: First Omagh- "A congregation of Omagh was formed in the 17th century from the earlier one of Cappagh known as Drumraw and Longfield. It was erected in 1676 and a meeting house built about Crevenagh."
The LDS don't seem to have any microfilmed records from any Presbyterian Church in Omagh in their catalogue.