A souterrain, as these structures are known, is a type of man-made underground chamber or tunnel, typically constructed during the early medieval period in Ireland, roughly between the seventh and twelfth centuries. Built from dry-stone walling and covered with large capstones, they were dug into the earth beside or beneath settlement sites, most likely used for cold storage, refuge, or both. The fact that one exists at Lissaniska places this otherwise unremarkable Mayo townland within a wider pattern of early Christian-era settlement activity across the island.

Souterrains are found in their hundreds across Ireland, and Mayo has its share, often associated with ringforts or other enclosed farmstead sites of the period. They vary considerably in scale and complexity, from simple single chambers to elaborate multi-roomed systems with ventilation shafts and deliberately low crawl-ways designed to slow any intruder. Without further detail specific to Lissaniska, it is not possible to say how large this example is, whether it remains intact, or what surface features might once have accompanied it. What can be said is that the presence of such a structure suggests this patch of ground was once part of a working early medieval landscape, farmed and inhabited long before the townland acquired its current name.