A souterrain is a dry-stone built underground chamber or tunnel, typically associated with early medieval ringforts and thought to have served as storage space, a place of refuge, or both. This one sits in the western quadrant of a ringfort, and its exposure is accidental rather than archaeological.

The passage runs east to west and measures at least 3.8 metres in length, 0.87 metres wide and a little over 0.67 metres high, constructed from dry-stone walling with flat stone lintels laid across the top. The walls appear to narrow as they run eastward, which may reflect the original design or the gradual settling of the structure over time. Ringforts, which are circular enclosures defined by earthen banks or stone walls, were the dominant form of rural settlement in early medieval Ireland, and souterrains are frequently found within them. At Clashganny, the structure has since had rubbish deposited inside it, an indignity that is unfortunately common for ancient features that become inadvertently accessible.