Though now sealed and inaccessible, archaeological surveys have revealed that this hidden structure consisted of at least two chambers running from east to west, carved into the earth just above where the land slopes down towards a small river.

Souterrains like this one were common throughout Ireland between the 6th and 12th centuries, typically built as annexes to ringforts or other settlements. These subterranean passages provided a cool, dry environment for storing food and valuables, whilst also offering a temporary hiding place during raids. The Cool Beg example follows the typical design, with multiple chambers that would have been accessed through a narrow entrance, making it easy to defend if necessary.

Today, only that modest stone marker hints at the complex underground architecture beneath the peaceful Donegal countryside. The souterrain forms part of a rich archaeological landscape documented in the Archaeological Survey of County Donegal, which catalogues field antiquities from the Mesolithic period through to the 17th century. While the passage itself remains closed to protect its structure, its presence serves as a tangible reminder of the ingenious ways Ireland's early inhabitants adapted their landscape for both practical needs and protection.