A two-and-a-half-acre scrap of land in Cork Harbour, it was once home to a purpose-built gunpowder magazine of considerable architectural ambition, and half of it is still there, more or less intact, while the other half was deliberately buried to make way for modern infrastructure.

The magazine was constructed between 1808 and 1818 to serve the Royal Navy's base at Haulbowline, which sits just across the water. Rather than a single utilitarian bunker, the designers produced two identical structures arranged symmetrically, each composed of three inter-connecting vaulted brick-built caverns opening off a flanking corridor. A shared forecourt separated the two halves, and the whole complex was entered from the north through an archway dressed in limestone ashlar with a classical surround, a formal gesture that seems almost incongruous for a building whose entire purpose was the safe storage of explosive powder. That formality reflects a broader Georgian naval tradition of imposing architectural order even on functional, sometimes dangerous, structures. When a road bridge was built across the island in 1966, the eastern section was filled in entirely. The western section remains open. Ancillary buildings have since been demolished, and the original fittings are gone. The island itself passed out of naval and state use sometime in the 1920s, sat empty for several decades, and was eventually purchased by Irish Steel in 1964, after which it was used for industrial waste storage, a considerable distance in purpose from its original role.

The bridge that crosses Rocky Island is part of the route connecting the Ringaskiddy ferry terminal area to Haulbowline, and the island can be glimpsed from it. Access to the structures themselves is another matter entirely, and the site's industrial history adds further complication to any casual visit. What remains of the western magazine, vaulted brick caverns and a classical entrance arch included, survives as an unusually refined remnant of Napoleonic-era military infrastructure, half-swallowed now by the twentieth century.