The structure that survives on a low north-south spur is not simply a ruined place of worship but a fortified church, a building type that fused ecclesiastical function with the defensive logic of a tower house. The pointed entrance in the south wall of the tower is guarded by a murder-hole, an opening in the ceiling of the lobby through which defenders could drop or pour unpleasant things onto unwelcome visitors. An oubliette, a concealed chamber tucked into the masonry of the vault, sits off the internal stair. There is also what appears to have been a garderobe, a wall-closet functioning as a privy, on the upper floors, with its chute still visible in the west wall. The building's internal arrangement is equally layered: the vault of the tower was incorporated into the body of the church and its first floor used as a gallery, creating a vertical complexity unusual in a rural parish church.

The documentary history of Killesk begins well before the church was built. The quarter fee of Killesk was held by one Auger de Ponte Chardun in 1247, and by the FitzGeralds or Barrons from 1324. A reference to the church itself appears in 1370, when the advowson, meaning the right to appoint its clergy, was granted to Dunbrody Abbey, the Cistercian house a few kilometres to the south-east on the Barrow estuary. The church was likely constructed around that period, though by the sixteenth century it may have been laicised, stripped of its formal religious function and absorbed into secular use. A nearby castle is documented from 1568. The tower's fabric reflects its hybrid origins: the quoins, the dressed corner stones, are of granite and conglomerate, carefully worked, while a slight batter at the base of the tower, a subtle outward lean designed to increase structural stability, is notably absent from the later eastern projection.

The remains sit beside a rectangular grass-covered enclosure, roughly 45 to 50 metres north to south and 32 metres east to west, defined by a low bank or scarp. Despite its churchyard-like form, no evidence of burial has been found within it. The tall windows that once lit the east wall and the ends of the north and south walls are gone, and the wall walk has not survived, but enough of the tower and its eastern projection remains to read the building's logic clearly, including the corbels that once supported the gallery floor and the alcove in the south wall from which the murder-hole was operated.