The structure is compact, roughly three metres by two and a half internally, with walls around sixty centimetres thick, and what survives includes doorways at both ground and first-floor level in the northeast wall, the arc of a low stone enclosure wall to the northeast, and the remains of a further structure pressed against the southeast wall. It is not dramatic in scale, but the detail is precise and deliberate.

The Ordnance Survey six-inch map of 1842 identifies this as Battery No. 4, set within what is marked as square ordnance grounds on Bear Island. A battery, in this military sense, is a prepared position from which artillery could be directed across a stretch of water, and Bear Island, sitting across the entrance to Bantry Bay, was considered strategically significant during the late eighteenth and early nineteenth centuries. The French had attempted a landing at Bantry Bay in 1796, and the threat of further invasion prompted a programme of coastal fortification along the southwest coast, including the construction of Martello towers, squat round defensive structures built to a broadly standardised design. The battery on Bear Island formed part of that same layered system, its gun loops allowing weapons to be trained on approaching vessels while the walls offered some protection to those manning them. The 1842 map placing it within ordnance grounds confirms it was still recognised as an active or at least administered military site at that date.