The structure is a circular redoubt, a self-contained defensive fortification designed to hold a garrison and resist attack from any direction, and it survives in a remarkably complete form. Its circular fosse, a defensive ditch running 7.5 metres wide and 8 metres deep, rings the whole complex, with a broad outer glacis, a gently sloping earthen bank intended to deflect cannon fire and deny cover to attackers, extending a further 36 metres beyond that.

Known on the Ordnance Survey six-inch map of 1842 as the West Redoubt, the battery was built by 1808, according to the military historian Paul Kerrigan, who places the date in his 1981 study of Irish coastal defences. It was designed to house a hundred men and to mount eight guns, positioned to command the western approaches to Bantry Bay. Inside the parapet wall, which stands 1.8 metres high and runs 4 metres thick, a level circular area some 58 metres in diameter contains two semi-circular ranges of rooms arranged around a central corridor 28 metres long. Each range comprises five single-storey barrel-vaulted rooms, the kind of robust, curved stone ceiling construction that could absorb the shock of bombardment, enclosed within a high stone wall reaching 6 metres. A well within the northern section of the parapet would have supplied the garrison with fresh water. The redoubt was not alone in guarding the island; two comparable fortifications were built further to the north-east along Whiddy Island as part of the same defensive scheme, reflecting the very real anxiety in Ireland about a French landing during the Napoleonic Wars, an anxiety with direct local precedent given the French fleet's appearance in Bantry Bay in 1796.

Whiddy Island is accessible by ferry from Bantry, and the south-western end of the island where the redoubt sits is sparsely populated. The structure itself occupies high ground, which made military sense in 1808 and makes for good visibility now; the concentric geometry of the earthworks is most legible when the ground vegetation is low.