It is, however, the best-preserved signal tower in County Mayo, and one of a chain of more than eighty such structures thrown up along the entire Irish coastline in a matter of years at the start of the nineteenth century. The tower stands within what was once a rectangular enclosure, now barely a ghost in the pasture, and roughly six metres to its west a circular grass-covered depression, about two and a half metres across, may be the socket that once held the signal mast itself.

The tower dates to around 1806 and was built by the British Board of Ordnance as part of an urgent coastal warning network. The fear was French invasion, and the solution was a continuous chain of signal stations running clockwise from Dublin Bay all the way around the Irish coast to Malin Head in County Donegal. Each station was positioned so that its naval signal post, a system of flags and semaphore-style apparatus, could be read by the next station in the chain. At Glash, the nearest neighbours were a station on the NW side of Achill Island, some 12.6 kilometres to the south, and another at the NE corner of the Belmullet Peninsula, roughly 20 kilometres to the north-north-east. Both are now ruinous, and neither is visible from the site today. The tower itself is a compact, two-storey rubble-stone structure, square in plan at about 4.2 metres internally, with the walls rising to their original height on most sides. Its east wall bows outward slightly to accommodate the chimney flue, and fireplaces with flanking alcoves served both the ground and first-floor rooms. The wooden floors and roof are long gone, though the joist-holes that once carried them remain visible in the masonry. The system was abandoned by the mid-1810s once the Napoleonic threat had receded.

The site sits on level ground towards the south-western tip of the Belmullet Peninsula, about 257 metres from the nearest coastline, with open views in most directions. A stone circle lies roughly 570 metres to the north-north-east, and a coastal promontory fort, a type of enclosure defined by earthworks or walls cutting off a headland, sits around 560 metres to the south-east, so the tower occupies ground that has been considered significant across very different periods. The earlier road running to the east of the site was most likely in use when the tower was constructed, though the minor road immediately to the north is a more recent addition, dating to after around 1910.