The tower on Ireland's Eye was given two. That small distinction sets this squat, circular fortification apart from every other example in north County Dublin, and it points to something specific: this was a two-gun tower, purpose-built to command both the anchorage and the approaches to Howth, the nearest harbour on the mainland. Martello towers, for those unfamiliar with the type, are thick-walled, round defensive structures developed by the British military in response to the threat of Napoleonic invasion. They were designed to be compact, hard to breach, and quick to man.

Construction across the Dublin coastal chain began in 1804 under the supervision of Colonel Benjamin Fisher of the Royal Engineers, and by December 1805 all towers were armed and complete. The Ireland's Eye example, recorded as Tower No. 3, was the largest of the north Dublin group. It sits on a rock outcrop at the north-western end of the island, and its scale reflects its dual armament, each gun being a 24-pounder. Unlike many towers along the Irish coast, no battery was constructed alongside it. The tower's architecture carries several details worth noting: a raised doorway on the south side with dressed limestone jambs, defended above by a machicolation, which is a projecting parapet with an opening through which defenders could drop material onto attackers below. There are small opes, or window openings, on the east and west sides. Perhaps most remarkably, it is the only tower in the group to retain all of its original 1805 Board of Ordnance boundary stones, the markers that formally delineated military land.

Ireland's Eye is an uninhabited island lying roughly a kilometre off the coast at Howth, accessible by seasonal ferry from the harbour there. The tower stands at the north-western end of the island, visible from the water as you approach. There is no formal infrastructure on the island, so visitors should come prepared for uneven terrain and variable weather. The boundary stones, low and easily overlooked, are scattered around the tower's perimeter, and knowing in advance that they are the sole surviving complete set from 1805 makes them considerably easier to appreciate once you are standing among them.