A souterrain is an underground structure, typically drystone-built, associated with early medieval ringforts and used variously for storage, refuge, or ventilation; they are common enough across Ireland, but many survive only as collapsed rubble or partial remains. The Lackan example is well-preserved, and its geometry is specific enough to be followed precisely: from the access point, one passage runs eastward for around five and a half metres, while another extends northward for nearly ten metres, reaching a maximum width of just under two metres. Off the northwest end of that northern passage sits a chamber running east to west, measuring roughly six and a half metres long, just over two metres wide, and one and a half metres high. An air-vent survives at the northeast end of the passage, a detail that points to careful, considered construction rather than simple excavation.

The souterrain sits within the southeast quadrant of a rath, the circular earthwork enclosure that was the standard form of settlement for farming families in early medieval Ireland. A second souterrain occupies the southwest quadrant of the same rath, which is a relatively unusual arrangement and suggests the enclosure was a substantial and well-organised site. In 1979, the Lackan souterrain became the setting for an experimental archaeological project, the findings of which were published by Fox in 1980, making it not merely a surviving monument but one that has been actively studied as a working structure. McCaffrey had noted it earlier, in 1952, so its presence in the local record stretches back over seventy years of documented attention.