No depression in the grass, no stone-lined entrance, no obvious break in the ground betrays it. The site exists primarily as a record of something that was once, briefly, more legible.

A souterrain is an underground passage or chamber, typically constructed during the early medieval period in Ireland, and usually associated with the ringfort, or rath, within which it sits. They served variously as places of refuge, cool storage for dairy produce, or escape routes. The one at Knockbrack was noted by Coleman in 1947, recorded under the neighbouring townland name of Knuttery, and described at that point as little more than an irregular hollow in the southern interior of the enclosing ringfort. Even that modest surface expression has since disappeared, leaving the souterrain as a feature known largely by inference and by the fact that someone thought to write it down.