The island is better known today for its walled garden and exotic plantings, but tucked near its summit is something considerably older and more militarily minded: the remains of a sub-rectangular enclosure whose walls still stand to roughly six metres in places, with a thickness of three and a half metres and the remnants of a wall walk along the top. These are not garden boundaries or field walls. They are the bones of a serious defensive structure.

The enclosure measures approximately twenty-four metres east to west and eight and a half metres north to south, and it sits to the east of a Martello tower, the squat, round coastal fortifications that the British built in large numbers around the Irish coastline during the Napoleonic Wars. The western wall of the enclosure was eventually replaced by a later, thinner wall, only around eighty-five centimetres thick and considerably lower than the earlier stonework, and it was this later section that physically connected the enclosure to the Martello tower. A date plaque reading 1815 survives above an arched doorway in this later wall, placing at least part of the construction firmly within that period of coastal anxiety, when the threat of French invasion shaped military infrastructure across Ireland and Britain alike. The 1815 date comes very late in that particular story, arriving the same year as Waterloo, which suggests the work on Garinish continued even as the strategic rationale for it was dissolving elsewhere.

The earlier enclosure, with its massive walls and wall walk, predates the Martello connection and points to a defensive use of this high ground that long preceded Napoleonic-era military planning. What exactly that earlier structure was built for, and by whom, is not recorded, but its scale alone, walls nearly twice the height and four times the thickness of the 1815 addition, suggests it was intended to be seen as well as used.