South Stack Lighthouse and former keepers' accommodation

South Stack Lighthouse and the former keepers' accommodation is a 28 metre high tower located towards the west of the island. The tower is constructed from masonry, using rock excavated from the site, and is rendered and painted white. It has a circular and tapering design. Bold modillion corbels support the projecting moulded parapet, which is topped with a cast-iron railing featuring fluted finials on the posts. The cast-iron lantern has lattice glazing and a domed roof with a cowl. There are projecting sills on the small stair windows and service room windows at the upper level, which are mullioned and transomed with raked-back upper lights.

Adjoining the tower to the east is the engine room and former keepers' accommodation, which is built from white-washed rubble and has two parallel slate roofs with coped east gables. This single-storey structure features an entrance in a projecting porch to the southwest and another in the east-facing gable, framed by a rusticated architrave. The windows are 4-light mullioned with external shutters.

Inside the tower, it is open up to the service room directly below the lantern, with a cantilevered stone staircase against the outer wall, which has a plain bellied cast-iron rail. A cast-iron tube runs the full height of the tower, which once housed the weights of the clockwork rotative mechanism. The optic is mounted on a cast-iron base, supported by cylindrical columns with square capitals and moulded brackets. The lens is six-sided and features prismatic reflectors that rotate in a bath of mercury. The engine room has light wrought-iron trusses on either side of a central cast-iron beam supported by cast-iron columns. The former keepers' accommodation consists of a series of rooms that open from a central corridor.