The passage extends to the south-west, measuring just 0.7 metres wide and 0.6 metres high, barely enough to crawl through. Its walls are built of drystone masonry, stones laid without mortar in the centuries-old tradition, and the ceiling is formed from heavy flat slabs placed across the top. Small as it is, this is a souterrain, an underground chamber or passageway typically associated with early medieval settlements in Ireland, thought to have served as storage space, a refuge, or both.

The souterrain sits within a rath, a type of enclosed farmstead common throughout early medieval Ireland, usually defined by one or more circular earthen banks and ditches. Raths were the standard unit of rural settlement from roughly the sixth to the twelfth century, and it was not unusual for their inhabitants to construct a souterrain beneath the enclosed ground, accessible only to those who knew exactly where to look. At Magherabrack, the access hole lies a short distance west of the rath's centre, placing it in a position typical of such features, tucked into the interior rather than near the perimeter. The association between the two monuments suggests they were part of the same agricultural and domestic complex, though precisely when either was built or by whom remains unrecorded.