The discovery prompted an archaeological investigation between 22 and 24 March, led by Richard Crumlish of Archaeological Services Unit Ltd. The site sits on a steep slope facing south-east, less than a mile from Greencastle village, commanding sweeping views across Lough Foyle.

The souterrain itself proved to be a substantial structure, built from uncoursed rubble and topped with flat stone lintels. Archaeological testing revealed it consisted of a main passage running west-north-west to east-south-east, with what appeared to be a chamber branching off to the south. Part of the roof had already collapsed, creating an opening 2.7 metres long and up to 1.2 metres wide, which allowed the team to peer inside. The structure sat perilously close to the house under construction, just 1.4 to 1.7 metres from its north-west wall. Seven test trenches were excavated across the site to determine the full extent of the underground feature and to check areas earmarked for water and sewerage services.

The investigation yielded modest but intriguing finds: a single pottery sherd from inside the souterrain awaiting analysis, three flints from the topsoil (one possibly worked), and scattered fragments of bone, including some tiny burnt pieces. The discovery had practical implications for the development; whilst the proposed sewerage route was given the all-clear, the water main needed rerouting along the north-east boundary to avoid the ancient structure. These souterrains, found across Ireland and Scotland, remain something of an enigma to archaeologists, likely serving as storage spaces, refuges, or perhaps holding ritual significance for early medieval communities.