Tamlaghtfinlagan Parish Church is in Ballykelly, Co. Londonderry, five kilometres west of Limavady on the Londonderry Road. The name means "The Plague Monument of Findluganus". Findluganus or Finlagan, Patron Saint of the parish, was a contemporary of and friend of Saint Columba. Columba is supposed to have founded an abbey in the district about 585.
In 1622, the church was in ruins. The Fishmongers' Company which had settled in the area after the Plantation, repaired and enlarged the old church, which was dedicated to St Peter. It was also known as the Garrison Church. This church was destroyed and restored twice during the 17th century. It was kept in good repair in the 18th century, with a chancel being built in 1719.
The Fishmongers also erected a castle at Walworth, west of Ballykelly. Walworth was named after Sir William Walworth who was Mayor of London at the time of Wat Tyler's revolt in 1381.
The present church in Ballykelly was built in 1795 by the Earl of Bristol, Bishop Hervey. The chancel, vestry and gallery were added in 1851, and the north aisle, by Joseph Welland, was built in 1859.
Set amongst trees in spacious grounds at the edge of the village, Tamlaghtfinlagan church is an impressive sight. It is a three bay hall with a tower and a tall elegant ashlar spire. There are crenellations
along the external walls. The nave walls are supported by buttresses.
The window in the west wall of the porch is in memory of Private Michael Boxall, 5th Co. Londonderry Battalion, Ulster Defence Regiment. Two Rolls of Honour in the porch commemorate those who fell in the Great War. The gallery is over the west end of the nave. The nave ceiling has a flat, classical design, and the ceiling in the sanctuary is vaulted. There are three windows in the south wall. Each has two lights and diamond panes with rectangular edges, and Y tracery and various crest insets. The middle window was installed in 1995 to mark the bicentenary of the church. On the left are inset illustrations of the church, 1795, of the rectory which was built in 1863, and of Jesus calling the children. On the right, are illustrations of the texts, "Go onto all the world" and "Do all to the glory of God", and St John's chapel at Myroe, 1863, is depicted.
The arms of the Beresford family also appear.