A rath is an early medieval enclosed farmstead, typically consisting of a raised earthen bank and an outer ditch, known as a fosse, surrounding a domestic interior. The one at Knocknaneirk measures approximately forty metres across in both directions, with a bank still standing to an internal height of 1.7 metres and a fosse cut to a matching depth of 1.7 metres. That symmetry, the bank and the ditch almost mirror images of each other in scale, gives some sense of the effort that went into its original construction.

The site carries a few small irregularities that make it worth closer attention. On the south side, a formal entrance six metres wide slopes gently downward toward the outside, a deliberate and practical piece of design that would have eased the movement of animals and people in and out of the enclosure. There is also a narrow gap in the bank to the north-west, less obviously intentional, and to the west-south-west the bank has been levelled, possibly the result of its material being pushed or tumbled into the adjacent fosse over time. Most unexpectedly, water runs along the base of the fosse on the eastern side, possibly fed by a spring, which would have made the site considerably more defensible or simply more liveable depending on how the original occupants chose to use it.

The interior today is dense with ferns and briars, and the north-east quadrant has been overtaken by young trees, so the earthwork is easier to read from the outside than from within. The fosse is shallower toward the south, worn down gradually, and the whole structure has that particular quality common to unexcavated raths across Ireland: visibly ancient, structurally coherent, but stripped of the domestic detail that only excavation or documentary record could supply.