A souterrain is an early medieval underground passage or chamber system, typically built from drystone, whose precise purpose is still debated but which likely served for storage, refuge, or both. What makes the one at Rathkenny particularly striking is the density of ogham writing associated with it. Ogham is an early Irish script consisting of lines and notches cut along a stone's edge, most commonly found on standing stones rather than tucked inside underground passages. Here, at least three ogham stones were incorporated into the structure itself, one still visible overhead on entering, and others used as lintels on the tunnel entrances to the side chambers. One of those lintels was removed without permission at some point, a loss that is noted with some frustration in the archaeological record.

The souterrain sits within a substantial multivallate rath known as Lismore or Lios Mór, a large ringfort defined by three concentric banks and ditches, with an overall external diameter of roughly 105 metres north to south and 106.6 metres east to west. It stands on rising ground with wide views of the surrounding countryside, the kind of position that would have made it both defensible and conspicuous. The zone between the inner and outer rings on the north-east through east to south side may have served as a working and living area, while the opposing arc appears to have been more heavily fortified. An opening was made into one of the souterrain's chambers in the late 1970s, which means the drystone construction is now clearly visible to anyone who enters. The centre chamber is partly backfilled from the way that opening was made, one of the side chambers was too flooded to investigate, and a tunnel running roughly north-east towards the fort's interior appears to have collapsed entirely, suggesting the underground network was once more extensive than what survives.