The feature is a souterrain, a type of artificial underground tunnel or chamber built during the early medieval period, typically associated with ringforts and cashels. They were used variously for storage, refuge, or concealment, and their builders went to considerable trouble to keep them out of sight. This one is no longer fully concealed. A section of the passage has collapsed, leaving an opening roughly four metres long and about a metre wide, sunk to a maximum depth of around 0.7 metres, its axis running east to west. On the southern side, some of the original drystone walling, stone laid without mortar, remains visible.

The souterrain sits within the northern half of a cashel, a type of stone-walled enclosure that served as a defended farmstead in early medieval Ireland. The cashel itself is a separate recorded monument. Underground passages of this kind were often integrated into such enclosures, their entrance concealed within a house or beneath the wall line, making them difficult to locate without prior knowledge of the site. What survives exposed at Meggagh represents only part of the original construction. There is a possibility that the passage continues further to the east, where a section measuring up to 7.5 metres in length and 1.5 metres in width may still exist underground, though this remains uncertain and has not been fully established.