Two concentric banks encircle the interior, separated by a fosse, the term for the ditch dug between them, whose spoil was typically thrown outward to heighten the outer bank. That outer bank survives along the south-west, west, and north-east arc of the monument, giving a clear sense of the enclosure's original scale and intent.

This is a rath, the most common type of ringfort found across Ireland, generally understood to be the enclosed farmstead of an early medieval family of some local standing, dating broadly to the period between the sixth and tenth centuries. The earthen banks were not primarily military fortifications in the way a castle wall would be; they defined territory, kept livestock in, and signalled status within a community. What makes the Killaghaun example worth attention is its state of preservation and the clarity of its causewayed entrance, a raised passage roughly three metres wide on the eastern side, where the fosse was bridged to allow access into the enclosure. Causewayed entrances like this are a recognisable feature of well-preserved raths, but seeing one so legibly intact is relatively uncommon.