The pairing is odd enough on its own, but the tower itself repays closer attention. Its exterior is coated in lime render pressed to imitate slate cladding, a small patch of the real thing still clinging to the upper north-east wall. The original entrance was on the first floor, reached by a retractable ladder, and is sheltered above by a machicolation, a projecting parapet designed to allow defenders to drop things on unwelcome visitors below. Small square bartizans, corbelled corner turrets, project from the north and east corners. Inside, the wooden floors and roof are long gone, but rows of joist holes in the walls mark exactly where they sat, and a square drainage channel cut into the north corner once carried rainwater down to storage barrels in the semi-basement below.

The tower was built around 1804 to 1806 by the British Board of Ordnance, one of more than eighty signal stations raised in the first decade of the nineteenth century to watch for a French invasion fleet. Each station communicated with its neighbours using a naval signal post, and together they formed a continuous coastal chain running clockwise from Dublin Bay all the way to Malin Head in County Donegal. The Inis Oírr tower sits at 65 metres above sea level with unobstructed views in almost every direction; in clear weather, the signal tower on Inis Mór is visible roughly 14 kilometres to the north-west, and the tower at Hag's Head, at the southern end of the Cliffs of Moher in County Clare, can be seen about 13.5 kilometres to the south-south-east. The chain was abandoned by the mid-1810s once the Napoleonic threat receded. Within the same enclosure wall, a mid-to-late nineteenth-century national school was later built just 13 metres from the tower, its two rooms, gabled porch, and red brick chimneystack appearing on the Ordnance Survey third edition map of around 1910. The enclosure also contained what appears to have been a toilet block, with a dividing wall suggesting separate playgrounds for boys and girls.

The tower stands about 143 metres south of Dún Formna, an oval stone cashel (a type of early medieval ringfort built in dry stone) enclosing a medieval castle, and roughly 500 metres from Teampall Chaomháin, the partly sand-buried church of St Cavan. The hilltop location means the approach from the main settlement on the northern shore involves a steady climb through a landscape of thin-soiled fields divided by tall drystone walls and stretches of bare limestone pavement.