That detail is the unusual part: the six-pronged iron support for the traversing platform, the mechanism that allowed a heavy cannon to swing in an arc and track a moving vessel, survives on its raised limestone plinth, along with the groove cut into the inner parapet to hold the iron "racer" on which the gun carriage wheels ran. Most Martello towers have lost these fittings entirely. Here, the rooftop artillery apparatus remains largely in place.

Martello towers were low, squat coastal fortifications built by the British across Ireland and England in the early nineteenth century, primarily in response to the threat of Napoleonic invasion. Their thick, curved walls were designed to deflect cannon fire rather than absorb it, and the guns were mounted on the roof rather than behind embrasures. The Rosslague tower is the most easterly of three that were positioned to guard the northern approaches to Great Island, a strategically significant anchorage. The tower measures 15.3 metres in diameter, with a slight flattening to the north-east and south-west giving it a subtly oval profile. The outer walls, 2.75 metres thick and faced in coursed limestone ashlar, slope gently inward and are finished with an oversailing coping at the top. Inside, the construction switches to brick. Entry is through a round-arched doorway set at first-floor level on the southern face, reached originally by a removable ladder, a deliberate security measure. A spiral stone staircase on the east side of the doorway descends to the ground floor, which is divided radially into four compartments around a central circular area. The room to the north-west, covered by a brick barrel vault, served as the magazine for storing powder and shot well away from any spark or flame. The first floor is divided into two rooms by a brick wall, each with a round-arched window and a fireplace, and the original wooden floor between the two levels has survived.