Somewhere beneath a north-west-facing slope in Grallagh, County Waterford, there are three small underground chambers that most people walking the land above would never know existed. A souterrain, to use the Irish archaeological term, is an artificially constructed underground passage or set of chambers, typically associated with early medieval ringfort settlements and used variously for storage, refuge, or ventilation of perishable goods. The one at Grallagh is a modest but technically varied example: three chambers connected by low crawl-ways known as creeps, each built using a different method, which is what makes it quietly interesting.

The souterrain was documented by J. P. McCarthy in 1978, having come to light during the 1970s. McCarthy's account, published in the Journal of the Cork Historical and Archaeological Society, recorded the three chambers in some detail. The first is earth-cut, roughly two metres by 1.7 metres and just over a metre high. The second was cut directly into bedrock, slightly smaller and with a variable ceiling height between 0.9 and 1.3 metres. The third was constructed using drystone walling, the technique of stacking stones without mortar, and measures approximately 1.6 by 1.1 metres. An earth-cut entrance passage, around 1.9 metres of which was visible at the time of recording, leads into the first chamber. The site sits within an earthwork, and a rath, the remains of a circular enclosure typical of early medieval farmsteads in Ireland, lies about 150 metres to the south, suggesting this was once part of a wider settlement complex.

The souterrain is now closed, and what a visitor would actually see today amounts to little more than a shallow, U-shaped depression in the ground where the entrance once was. The underground structure itself is no longer accessible.