The tower no longer looks like much from a distance, but it was built in around 1806 as one link in a chain of more than eighty signal stations constructed by the British Board of Ordnance along the entire Irish coastline, running clockwise from Dublin Bay to Malin Head in County Donegal. Signalling between stations was accomplished using a naval signal post, a system of flags and apparatus borrowed from maritime practice. By the mid-1810s, with the Napoleonic threat diminished, the whole network was abandoned, and the towers began their slow return to the landscape.

What survives on Knocklackan is the semi-basement level and fragments of the ground floor, built from randomly coursed rubble stone. The original building was a square plan of about 4.25 metres internally, with its walls oriented to the cardinal directions. The east wall preserves the most legible detail: a bulge on its outer face marks where a chimney flue once ran, and a square-profile chute cuts through the full width of the wall near the north corner, opening internally at the base of a square-headed alcove beside a fireplace. Slot holes in the north and south walls show where the floor timbers once sat. The original doorway, based on the surviving evidence, would have been set at first-floor level on the west face, with small bartizans, projecting corner turrets, positioned over the north-east and south-east corners. The tower was set onto a rock-cut terrace carved into the hillside, and the curious near-absence of fallen rubble around the building suggests the stone was systematically removed after abandonment, likely put to use elsewhere on the island. About eleven and a half metres to the north-west sits a small oval lime kiln, a structure used to burn limestone to produce quicklime for mortar or agricultural use, already present when the Ordnance Survey mapped the area in the late 1830s.

The tower's position just east of Knocklackan's 192-metre summit means it has wide views to the north, east, and south, though the summit itself blocks the west and a neighbouring hill interrupts the south-east sightline. In clear weather, the partially-ruined signal tower at Shivel Head on Clare Island is visible some 11 kilometres to the north-north-east, an indication of how the chain was meant to function. The Cleggan Hill tower in County Galway, roughly 15.5 kilometres to the south, can no longer be seen from the site. Closer at hand, and easy to overlook amid the blanket bog and turf-drying stands, a concrete trigonometry pillar stands about six and a half metres to the north-east of the tower, ringed by a small modern cairn, a quiet layering of one era's surveying upon another's.