This is the Centre Redoubt at Reenavanny, one of three circular defensive works arranged along the island's spine, and it is among the more complete examples of early nineteenth-century coastal artillery infrastructure in the south of Ireland. A redoubt is a self-contained defensive enclosure, typically circular or polygonal, designed to hold out even if surrounding positions fell. Here, the engineering is still legible in the landscape: a fosse, the wide defensive ditch ringing the position, runs to around 8.5 metres across and drops roughly ten metres in depth, with an outer glacis, a gently sloped earthwork bank intended to deflect cannon shot and complicate an attacker's approach, extending a further forty metres beyond it.

According to the military historian Paul Kerrigan, the redoubt was complete by 1806, built to garrison 150 men and carry twelve guns. The context is the Napoleonic Wars, when Bantry Bay was considered a likely invasion corridor; French forces had famously attempted a landing there in 1796. Inside the fosse, a parapet wall encloses a level circular area sixty-eight metres in diameter, and along the south-western section of that parapet, low curved arcs mark where the gun emplacements once sat. At the centre of this space, three ranges of single-storey barracks were constructed, all barrel-vaulted in stone and originally finished with hipped slate roofs. The central range is the widest at nearly thirteen metres, flanked by two narrower ranges on either side. A covered well to the south-east of the central range retains its original iron pump. One of the ranges has, at some point since the garrison departed, been converted into a private residence.

Whiddy Island is accessible by ferry from Bantry, and the presence of a residence within the redoubt itself is a reminder that this is not an uninhabited ruin but an active part of a small island community. The three redoubts form a chain across the island, and walking between them gives a clear sense of how the defensive logic worked: each position covered ground the others could not, with Bantry Bay visible in nearly every direction.