The one on Haulbowline Island is different. It occupies the northern edge of an island in Cork Harbour that is home to the Irish Naval Service, which means the tower has spent its post-Napoleonic life inside a working military installation rather than slowly weathering on a clifftop for walkers to admire. That inaccessibility has also helped preserve something of its original character, including its coursed limestone walls and, at least as of recent years, a restored wooden floor.

Martello towers are squat, thick-walled defensive structures, usually circular or oval, built across Ireland and Britain in the early nineteenth century in response to the threat of Napoleonic invasion. The Haulbowline example was constructed between 1813 and 1815 at a cost of £3,000, and its proportions are notably substantial: oval in plan at 14.6 metres by 9.1 metres, and standing roughly 13.4 metres tall. Like others of its type, the entrance is set at first-floor level rather than at ground, a deliberate defensive feature that made forcing entry considerably more difficult. A large water tank was installed in the roof area, suggesting the tower was intended to function as a self-contained post for some time if needed. It stood on what was then designated the Ordnance Ground, the portion of the island given over to military stores and infrastructure.

At the time this record was last updated, the Naval Service was in the process of converting the tower into a museum, a fitting second life for a structure that has spent two centuries at the centre of Ireland's principal naval anchorage. Whether that work has since been completed, and what access if any is available to visitors, would require checking directly with the Naval Service, given the island's restricted status.