The one at Belvelly, sitting on a narrow, low-lying spit at the north-western corner of Great Island in Cork Harbour, was no exception. Yet in 1867 a band of Fenians successfully attacked it, which is not the kind of event that tends to appear in the official history of structures designed to be militarily impregnable. Locally it goes by the name Monning Tower, and that local identity, separate from its military designation, hints at the degree to which it became woven into the life of the island rather than remaining a purely imperial fixture on the waterline.

Martello towers, squat circular fortifications built across Ireland and Britain from the early nineteenth century in anticipation of Napoleonic invasion, were designed for both resilience and self-sufficiency. Monning Tower follows the standard pattern in broad terms but has its own particular character. It measures 15.5 metres in diameter and is built of coursed limestone ashlar, with a noticeably flattened profile to the east and west. Entry is through a first-floor doorway on the south side, beneath a rounded arch, with a spiral stone staircase giving access both down to the ground floor and up toward the roof, though the steps are now broken above first-floor level. Inside, the ground floor is divided into four radially arranged chambers around a central circular area; the northernmost of these served as a magazine, sealed by a brick vault that has since collapsed. The inner face of the tower and the dome above the first floor are of brick rather than stone. Two first-floor windows, each widely splayed with round-headed arches, pierce the east and west walls, and each is flanked by a fireplace, a detail that gives the interior an unexpectedly domestic quality for a fortification. The wooden floor between the two levels is gone. This was one of three Martello towers positioned to guard the northern approach to Great Island, the others standing at recorded positions along the same coastline.

The Fenian attack of 1867 is the episode that sets Monning Tower apart from its two neighbours. The Fenian Brotherhood and the Irish Republican Brotherhood were at that point mounting an insurrection across Ireland, and a garrisoned coastal tower was a plausible target. That the attack succeeded is the curious detail; the tower was taken, if briefly. It is a small, strange footnote to a wider rebellion, and it happened here, on this particular spit of land in Cork Harbour, inside a building whose broken staircase now prevents anyone from reaching the roof.