It is, in fact, the remains of a military redoubt, a compact defensive fortification typically built in a roughly circular plan to allow all-round fire, and it was once equipped to garrison a hundred soldiers and mount eight guns.

The battery at Reenavanny was one of three such redoubts constructed along Whiddy Island, all of them broadly similar in design. Built by 1807, the three positions, designated the East, Middle, and West Batteries, were spaced along the island to command the waters of Bantry Bay. The Reenavanny structure is comparable in scale to the West Battery and shares its layout with the Middle Battery, suggesting a coordinated programme of construction rather than piecemeal fortification. The timing places their origins firmly in the Napoleonic period, when Bantry Bay, already infamous in British and Irish military memory following the French fleet's near-landing in 1796, remained a serious strategic concern. An aerial photograph taken by Daphne D.P.C. Pochin Mould recorded the Reenavanny site still recognisable from the air, with three internal ranges visible at the centre despite the encroaching vegetation.

The site is currently inaccessible, and the combination of overgrowth and its island location means it survives more as a geographical curiosity than a place one can readily visit. Its very obscurity, reduced on a Victorian map to the ambiguous outline of a field, is part of what makes it quietly arresting: a garrison position designed for a hundred men, eight guns, and a French invasion that never quite came, slowly becoming indistinguishable from the landscape around it.