This underground passage, roughly aligned from north-northwest to south-southeast, stretches just over five metres in length and demonstrates the remarkable engineering skills of its builders. The structure narrows dramatically along its course; starting at 1.1 metres wide at the southern end, it constricts to just 60 centimetres about four metres along, before widening again to 1.4 metres at the northern terminus, where the wall has been carefully bonded into the side walls.

The souterrain's interior reveals intriguing architectural details that hint at its original purpose. A small recess, measuring 40 by 50 centimetres, has been carved into the southern wall, whilst the eastern wall contains a mysterious small passage positioned 28 centimetres above ground level. Though this secondary passage measures 40 centimetres square, its interior has unfortunately collapsed over the centuries, leaving its full extent unknown. The main chamber maintains an average height of approximately one metre throughout, roofed with sturdy flagstones that have largely survived intact, save for a gap at the southern end which now serves as the primary entrance.

The location of this ancient structure is particularly noteworthy, situated on fertile land that slopes gently southward towards the River Swilly. Souterrains like this one were typically constructed between the 6th and 12th centuries AD and served various purposes; from food storage and refuge during raids to ritual or ceremonial uses. The Ballymacool example, with its careful construction and strategic positioning on good agricultural land near a water source, suggests it played an important role in the daily life of the early medieval community that once inhabited this area.