What makes it quietly arresting is not drama but precision: surveyors found the site prominent enough to place a trigonometrical station on its north-eastern bank, using the ringfort's own modest height as a reference point for mapping the surrounding countryside. The monument, in other words, was still doing useful work in the nineteenth century, long after the people who built it were forgotten.

A ringfort, sometimes called a rath, is an enclosed farmstead of the early medieval period, typically dating from roughly 500 to 1000 AD. The enclosure here was recorded on the Ordnance Survey six-inch map of 1840 as a circular fort, and again on the twenty-five-inch edition of 1897 as a circular embanked enclosure with an external diameter of forty metres. When the Archaeological Survey of Ireland inspected the site in 2000, they found a raised circular interior of thirty metres in diameter, enclosed by an earth and stone bank roughly four and a half metres wide, with an external ditch, or fosse, of similar width running around the outside. The entrance gap, two metres across, faces south-south-west. A later field boundary, running north-east to south-west, has been built along the north-western edge of the fosse, showing how subsequent generations of farmers quietly absorbed the older landscape into their own working one. A second ringfort lies about 150 metres to the west, which suggests this corner of Killonahan was once a settled and organised place.

The site sits close to a cluster of townland boundaries, roughly 150 metres north of Rathbranagh, 90 metres east of Garranroe, and 100 metres west of Ballylusky, which helps with orientation if you are approaching across farmland. The interior was described in 2000 as level, dry, and overgrown, so sturdy footwear is sensible. The earthworks are low, the bank rising less than a metre externally, so the enclosure reads best from a slight distance rather than from within it. The site is visible on Google Earth imagery, which can be useful for understanding the full circular shape before you visit, since ground level can flatten what the aerial view makes obvious.