That one act of agricultural pragmatism has left an oddly legible record of time passing: a cashel, one of Ireland's dry-stone circular enclosures used as a defended farmstead, bisected by a field boundary that postdates it, surrounded by cultivation ridges that somebody ploughed long after whoever built the original walls had gone. Lough Gur sits just 200 metres to the south-east, and the views from up here extend in every direction, which rather underlines the point that this was not an accidental place to build.

The structure was first recorded cartographically on the Ordnance Survey 25-inch map of 1897, having been absent from the earlier 1840 six-inch edition, though its absence from the earlier map likely reflects the limits of what surveyors recorded rather than any change on the ground. The site was identified as a cashel or stone fort by the archaeologist M. J. O'Kelly in 1944, again by Seán P. Ó Ríordáin in 1954, and once more by O'Kelly in 1978. The Archaeological Survey of Ireland described it as a broad oval, flattish outcropping area with external dimensions of 44 metres north to south and 34 metres east to west, defined by the base of a stone bank. On its southern side, a rock-cut fosse, essentially a defensive ditch cut directly into bedrock and roughly five metres wide, adds a further layer of deliberate construction. Aerial photographs taken by the Cambridge University Collection of Aerial Photography in July 1966 confirm the site's outline clearly from above, as do successive satellite images taken between 2006 and 2020.

The cashel does not sit in isolation. Within roughly 300 metres of it there are two enclosures to the north-east, two hut sites to the east, a ring cairn further to the east-north-east, and another ringfort to the south, making Knockfennell Hill something closer to a small, densely occupied ancient landscape than a single monument. The rocky pasture makes the going uneven underfoot, and the exposed summit means the weather can shift quickly. The interior of the cashel, once a defined and enclosed space, is now crossed by that later field boundary running east to west, so reading the original layout takes a moment's patience. Looking towards Lough Gur from the western summit helps orient the whole picture, placing this particular enclosure within the broader archaeological concentration for which that area of Limerick is well known.