Gun loops punctuate the first-floor wall, narrow openings designed to allow a defender to fire outward while remaining largely protected, and a chimney stack survives in the northeast corner, a small domestic detail inside what was clearly a military building.

The 1842 Ordnance Survey map identifies it as Battery No. 3, one of several fortified positions laid out in rectangular ordnance grounds on the island. It sits roughly half a kilometre northeast of a Martello tower, the squat, round defensive structures built across Ireland and Britain during the Napoleonic Wars period to guard coastlines against potential French invasion. Bear Island, commanding the entrance to Bantry Bay, was a strategically obvious place to fortify: the bay had already seen a French fleet attempt a landing in 1796. Battery No. 3 was part of a coordinated defensive network rather than an isolated outpost, with access provided through a ground-floor and first-floor door in the west wall. To the southeast of the main structure, two arcs of low stone walls survive, their precise function unclear but likely connected to the wider ordnance complex.

The enclosure that once bounded the ordnance grounds is now largely collapsed, and the battery itself has lost its roof, leaving the interior open to the sky. What remains is compact but legible: a small fortified room with thick walls, firing positions, and a hearth, marooned in a field on an island that still bears the layered marks of its long military history.