The pivot is a small but telling detail: it is the socket around which a cannon on a wheeled gun carriage would have rotated to sweep a full arc of fire across the surrounding water. The tower itself is compact and deliberately massive, with walls nearly two and a half metres thick and an internal diameter of just over six metres, the kind of proportions designed to absorb artillery fire rather than simply resist small arms.

Martello towers were a coastal defence measure introduced across Ireland and Britain during the Napoleonic Wars, when the threat of French invasion gave military planners reason to fortify exposed shorelines. The Garinish example is part of that programme, though the site is more complex than a simple tower on a hilltop. To the east of the tower lies the remains of an earlier sub-rectangular enclosure, its surrounding stone wall still standing to around six metres in places and thick enough, at over three metres, to suggest a substantial and deliberate defensive presence that predates the Martello construction. A later, thinner wall replaced the western side of this enclosure and now connects it directly to the tower; above its arched doorway, a date plaque reads 1815, giving at least one fixed point in the site's construction history. The ground floor of the tower has been infilled over time, but the first floor retains its splayed window openings, a fireplace set into the southern wall, and a brick-vaulted roof overhead. Spiral stairs built into the northern wall lead up to the paved roof area, enclosed by a parapet with a gun carriage track running along the wall walk.

Garinish Island is perhaps better known elsewhere on its shoreline for the ornamental gardens developed in the early twentieth century, which draws most visitor attention. The tower, sitting apart on its elevated ground and connected to the older enclosure by that 1815 wall, occupies a different register entirely, a layered military site where two distinct phases of defensive thinking are still legible in the stonework.