What makes it stranger still is its biography: built around 1804 to 1805 as a military watchtower scanning the Atlantic for French warships, damaged by fire, then quietly repurposed as domestic accommodation for lighthouse keepers, and eventually left to outlast the lighthouse it served. The tower sits within a walled enclosure alongside the ruins of that lighthouse and a scatter of associated single-storey structures, all of them now roofless or collapsed, the whole compound accessible by a roadway from the west.

The tower was one of more than eighty signal stations built by the British Board of Ordnance along the Irish coastline during the first decade of the nineteenth century. The system ran in a continuous coastal chain from Dublin Bay clockwise around to Malin Head in County Donegal, with signalling carried out using naval signal posts. The Clear Island station's nearest neighbours in the chain were at Ballylinchy on the mainland to the north-east, roughly ten kilometres away near Baltimore, and at Knock to the north-west, about twelve kilometres distant, where the tower survives in an altered form inside a modern dwelling. The Clear Island tower itself was completed by 1805, according to Paul Kerrigan's research, and the whole network was abandoned by the mid-1810s once the threat of French invasion had receded. After the fire, the building was converted around 1818 into accommodation for keepers of the adjacent lighthouse, which operated until about 1854 when it was decommissioned following the establishment of a new lighthouse on Fastnet Rock. The present Fastnet lighthouse, one of the more recognisable structures on the Irish coastline, replaced even that one, having been built between 1899 and 1903.

The tower's fabric rewards close attention. Built from roughly coursed rubble sandstone, it retains sections of its original lime render and weather-slating on the exterior. The original entrance was at first-floor level on the south-west elevation, reached by a retractable ladder and protected by a small machicolation, the projecting stone box above a doorway through which defenders could drop objects on anyone attempting to force entry. That doorway was later blocked, replaced by a ground-floor entrance with a concrete lintel, which was itself subsequently blocked. Most signal towers of this period and type carried small bartizans, the corbelled corner turrets projecting from the parapet, at their rear corners; this one has none, and the masonry at those corners appears to have been neatly repaired, suggesting they may once have existed and been removed. About two hundred metres to the north-east, the early Ordnance Survey mapping marks a signal pole at an elevation of 448 feet. The island also holds a passage tomb roughly a kilometre to the south-south-west, and Doonamore Castle, a tower house on a narrow coastal promontory, about two kilometres to the west.