It sits roughly 100 metres from the cliff edge, fenced off since 2020 due to structural damage at its base, and it looks, at first glance, like a fortified farmhouse that somehow ended up in the wrong place. In fact it was a military signalling station, completed by 1805, and its job was to watch for a French invasion fleet that never came.

The tower was one of more than 80 such stations built by the British Board of Ordnance in the early nineteenth century, forming a continuous coastal chain running clockwise from Dublin Bay all the way to Malin Head in County Donegal. Each station communicated with its neighbours using a naval signal mast, relaying warnings along the coast at speed. The Ballylaan tower sat within this chain between a station on Inis Oírr on the Aran Islands, roughly 13.5 kilometres to the north-north-west, and one on the west side of Mutton Island, about 15.9 kilometres to the south-south-west. Architecturally, the tower is a two-storey limestone rubble structure, roughly 5.85 metres square, with block-and-start quoins at the corners and a first-floor doorway that would originally have been reached by a retractable ladder, a deliberate defensive measure. A machicolation, a projecting parapet above the doorway designed to allow defenders to drop objects on anyone below, sits over the entrance supported by three cut stone corbels. Two small windows flanking the doorway appear to be unique to this site among the tower series, departing from the standard design used elsewhere. At the east and south corners, bartizans, small overhanging turrets also corbelled out from the wall, add to the tower's defensive character. The whole system was abandoned by the mid-1810s once the Napoleonic threat had faded.

What makes the site still stranger is its layering of periods. The tower was built within a badly eroded promontory fort, a type of prehistoric or early medieval enclosure that uses natural cliff edges as its defences, and immediately to the south-west the low remains of a Second World War lookout post, L.O.P. 47, survive in the grass. Three different moments of coastal watch, separated by centuries, occupy the same exposed headland just east of Hag's Head.