It was not a castle. It was a signal tower, built in haste around 1804 to 1805 as part of a coastal warning network, and by the time the surveyors came through in the 1840s it had already been reduced far enough that the romantic label seemed to fit. By the second survey, carried out between 1893 and 1897, it had disappeared from the maps entirely, the masonry carted off for use elsewhere, leaving little more than a faint depression in the ground and a low earthen bank.

The structure was one of more than eighty signal stations erected by the British Board of Ordnance along the Irish coastline in the early 1800s, each positioned to relay warnings of an approaching French invasion fleet using a naval signal post. Together they formed a continuous chain running clockwise from Dublin Bay around the entire coast to Malin Head in County Donegal. The Knocknagarhoon tower, a two-storey square building roughly 5.85 metres on each side and built from roughly coursed rubble, sat 126 metres above sea level on a small hill with clear views in all directions, the Clare coastline some 440 metres to the north-west. Its nearest neighbours in the chain were at Loop Head, about 12.4 kilometres to the south-west, and at Ballard, around 14.3 kilometres to the north-east; both are also now demolished. The whole system was abandoned by the mid-1810s once the threat from Napoleonic France had faded, leaving towers scattered around the coast that locals would repurpose, rename, or simply forget.

What remains at Knocknagarhoon today is largely invisible as architecture. A semi-circular area of overgrown disturbed ground, roughly five to six metres across, marks where the tower stood, enclosed on most sides by a low earthen bank no more than sixty centimetres high. A little stone rubble is still present. The place sits within ordinary enclosed pasture, defined by the rubble stone walls and earthen banks typical of the region, and there is evidence of small-scale quarrying nearby, which may account for some of the missing stonework. It is a site that rewards the habit of reading the ground rather than looking for walls.