Derg, "The Oak Wood", is a parish which is centered in the west Tyrone town of Castlederg. The ancient church was reported to be in ruins in 1619, when it was rebuilt by Sir John Davis. This was destroyed in 1641, and it remained in ruins until 1731 when the nave of the present church was built by Hugh Edwards of Castle Gore.
It is certain that the tower at the west end of Derg church is older than the nave, as it is more Elizabethan or Jacobean in architectural style. It presumably survives from the old church. It has a fine classical doorway which is flanked by free standing Tuscan columns, and it is surmounted by a high pediment with a crest carved upon it.
Inside the porch, the entrance door to the nave is surmounted by a round-headed archway with the Latin inscription "auspiciis Domini resurgo", which can be translated, "by the favors of God, I rise", and the date is 1731. The architrave has floral patterns and acanthus leaves on an ogee moulding. The door is in memory of William and Fanny Harper, 1959. The sections of paneling in the porch were donated in memory of Mary Jane Bogle, Samuel and Rebecca Loughlin, Joseph and Elizabeth Milligan, Jack Cummings, Jack Cooper, Harry Dougal RN, and Samuel and Lilian Walls. The tiling on the floor of the porch was the gift of Colonel and Mrs Carr, 1971, the floodlighting is in memory of Ronald Finlay and the retaining wall in the churchyard is in memory of Leslie McNutt, 1992. To the left of the porch is the Smyly Rooms which was built in 1971 to commemorate John Denis Ferguson Smyly who died in 1941, and his father, uncle, and grandfather.
The window over the west entrance to the nave depicts the Lamb of God. The nave is of five bays, and to the left is the north aisle. They are joined by four arches. On the south nave wall, the first window which is the baptistery, depicts St. John, and the second is of square-paned opaque glass with an inset depicting St. Luke. The third window was donated in 1959, and shows Jesus blessing the children. It is in memeory of Andrew McCready who died in 1957. The fourth window depicts St. Mark and the fifth, St. Matthew. In the west aisle wall there is a round window, and there are five windows with opaque glass in the north aisle wall. The window in the east aisle has three lights and tracery. The east window has three lights of diamond colored glass, and the middle section has an inset depicting the Lamb of God.
The baptistery is in the south-west corner of the nave. The font was presented by the children of the Sunday School in 1931. There is a stone pulpit on the right side, and a prayer desk on the left. The Conacher organ has two manuals and pedals. The blower was presented by the Mothers Union in 1954. In the sanctuary, the panelling on the south side commemorates those who fell in the Great War, and that on the north side, those who fell in the Second World War.
On the south wall, a monument commemorates the Rev. Edward Edwards, JP who died in 1881. He was perpetual Curate of Derg, 1849-1880. Adjacent is an inscription in his memory which was erected by his daughter. On the same wall, a monument commemorates Lt. Col. Edwards and Lt. Edwards who died in 1884. It is surmounted by a crest. There is also a memorial to William King Edwards, DL who died in 1912. On the west wall there is a brass memorial to those who were killed whilst serving in the Royal Ulster Constabulary and with the Ulster Defence Regiment. There is a second record of the donation of £300 to the church by Major John Smyly in 1896. On the north wall, a memorial commemorates Sir Robert Ferguson Bart., MP for Londonderry, who died in 1860, and other members of his family. On the aisle north wall, the Rev. Archibald Hamilton, perpetual Curate of Derg, 1808-1849 is commemorated along with other family members.