A souterrain, to use the technical term, is an artificially constructed underground gallery, typically stone-built, associated with early medieval ringforts across Ireland. They served various purposes, probably including storage and refuge. Most are relatively simple in plan. The one at Liscahane is not. Its sequence of two right-angle turns and one U-shaped reversal divides the gallery into four sections, with the entrance so narrow, barely half a metre wide, that a person would have had to squeeze through it sideways. The U-shaped creepway connecting the first and second sections is just fifty centimetres high. Whatever the structure was designed to protect, it was not meant to be easily reached.

Archaeologist B. Ó Donnabháin excavated the site over three seasons between 1982 and 1984, and the findings were considerable. The construction trench, cut to an average depth of 3.5 metres below the modern ground surface, yielded charcoal samples that produced radiocarbon dates centring on the sixth century AD, placing the souterrain firmly in the early Christian period. The gallery itself is roofed with flat stone lintels throughout, and two of those lintels, both from the steeply sloping second section, carry ogham inscriptions. Ogham is an early medieval script in which letters are represented by groups of notches and lines cut along a central stem, most often seen on standing stones; finding inscribed examples reused as building material underground is less common and raises its own questions about where those stones originally stood and what happened to them. The excavation also revealed that the souterrain's construction had disturbed an Early Bronze Age burial, meaning the site had already been significant to people living more than a thousand years before the underground passage was ever dug.