At some point after the rural electrification of the area, a work crew digging a hole for an electricity pole broke through the ground and found empty space beneath. What had opened up was a souterrain, an underground stone-lined passage or chamber, typically constructed during the early medieval period and associated with nearby settlements as a place of refuge or storage. No one had been looking for it.

The souterrain lies in the northern half of a ringfort, a type of enclosed farmstead common across early medieval Ireland, consisting of a roughly circular area bounded by one or more earthen banks and ditches. The ringfort at Lisnagat was presumably the homestead to which the souterrain belonged, a not uncommon pairing; souterrains were frequently built within or adjacent to ringforts, and their passages could sometimes be accessed from inside the enclosure. Beyond the circumstances of its discovery and its position within the ringfort, very little else is documented about the Lisnagat souterrain.

There is nothing to see at the surface today. No masonry is visible, no obvious depression marks the ground, and the site leaves no impression on the landscape for a visitor to read. Its existence is known only because the earth briefly gave way under modern utility work, offering a momentary glimpse of something old before, presumably, the hole was filled and the pole went in.