Their trunks push up through gaps in the stone roof lintels, which have long since shifted apart, and in doing so they mark out the line of a structure that was built to be invisible, to conceal and protect, and has instead become gradually, quietly visible again over the centuries.

A souterrain is an artificial underground passage, typically associated with early medieval settlement in Ireland, and used variously for storage, refuge, or escape. The example at Meggagh runs for just over seven metres on a roughly north-south axis, and its construction is a study in two different approaches joined together. The northern section, which lies within the fort itself, is cut directly into the bedrock, its floor rising toward the north as the ground inside the fort rises, its passage narrowing as it goes. Four lintels cover it, increasing in height to follow the slope. The southern section, which extends beyond the fort and down a steep rocky incline, is free-standing rather than rock-cut, built instead from large flat stones roughly a metre long, with four roof lintels still in place. At the northern end, an uncovered length of the passage slopes upward to ground level, and this open section may represent what remains of the original entrance. The souterrain sits within, and partly spills beyond, the southern edge of a cliff-edge fort, a type of promontory enclosure that uses the natural boundary of a cliff as part of its defensive circuit. That the underground passage extends over the steep slope outside the fort's boundary adds an unusual dimension; it suggests the souterrain was conceived as something that could move a person, or goods, beyond the defended perimeter and away.