That detail, passed down from a landowner's father, is one of the few human-scale measurements we have for what lies under the stone. The boulder itself, roughly square in cross-section at 0.8 metres by 0.8 metres, seals what is almost certainly a souterrain, an underground stone-lined passage or chamber typically built during the early medieval period, often in association with raths. A rath, also called a ringfort, is a circular enclosure defined by an earthen bank and ditch, used as a farmstead or settlement. Souterrains within them served variously as cold storage, refuge, or concealed escape routes.

The structure at Gortalassa sits just inside the north-north-western bank of the rath, and the loose stones visible in the hollow, some of which appear to form drystone walling, suggest the underground chamber retains at least partial integrity. The site was noted as far back as the 1840s, when Ordnance Survey records described a large cavern on the western side of the rath, with a covering stone of about twelve feet in length lying flat over the entrance. That measurement corresponds closely to what surveyors found later, suggesting the site has remained largely undisturbed. Carved faintly into the south-south-western end of the boulder are four symbols: a cross, a sword, a harp, and a shamrock. These are not ancient. According to the landowner, a local man added them in the early years of the twentieth century, which makes them a relatively recent and quietly personal addition to a stone with a far longer history beneath it.