This underground passage, stretching approximately 12.5 metres in length, curves gracefully from northwest to southwest from its current entrance. The structure showcases the impressive engineering skills of its builders, with dry stone walls carefully constructed without mortar and a sturdy flagstone roof that has weathered centuries. The main passage reaches heights of up to 1.3 metres, though it comes to an abrupt end at its southwestern terminus, where it appears to have been deliberately blocked off at some point in its history.

What makes this souterrain particularly fascinating is its complex internal layout, featuring multiple chambers branching off from the main passage. At the southeastern end, visitors would have found a side chamber measuring roughly 2.25 metres long and 1 metre wide, with a ceiling height of 1.1 metres. Along the eastern side, a larger second chamber extends about 3 metres in length and 1.25 metres wide, boasting the most generous headroom at 1.4 metres high. This chamber connects to a smaller antechamber to the southeast, measuring approximately 1.25 metres by 0.75 metres, creating an intricate underground network that likely served multiple purposes for its ancient inhabitants.

The souterrain's location was clearly chosen with care, situated on good quality land that slopes southwestward towards a nearby stream. This strategic positioning would have provided both practical advantages for water access and defensive benefits. These mysterious underground structures, found throughout Ireland but particularly common in Ulster, are thought to date from the early medieval period and likely served as storage spaces for food and valuables, refuges during times of conflict, or perhaps held ritual significance for the communities that built them. The Gortleck example, with its multiple chambers and substantial construction, represents one of the more elaborate souterrains documented in County Donegal's archaeological survey.