Holy Trinity Church, known locally as Boyd's Church, stands in the Diamond, Ballycastle's main square. Colonel Hugh Boyd paid for it in full and it was completed in 1756, in a Graeco-Italian style with a curved chancel and an octagonal spire about 30 metres high. For many years it was effectively the Boyd family's private chapel, and generations of Boyds lie in the vaults beneath it. It was given to the Church of Ireland around 1950.
The church and the rest of the Diamond are listed Grade A, and the square is the heart of the Ould Lammas Fair each August. The church is open daily from 9am to 5pm.