This one leaves nothing at all. It exists only as a pattern of magnetic disturbance, detected beneath a pasture on the south-facing slope of Knockroe Hill and recorded in a geophysical survey report from 2008. No outline appears on any Ordnance Survey map. No surface trace shows up on aerial photography taken between 2005 and 2012, nor on satellite imagery. What is known about it comes entirely from instruments rather than eyes.

The survey, a fluxgate gradiometer survey carried out by Cleary in 2008, identified the feature as number 7 within Area 2 of the investigation. A fluxgate gradiometer is a type of magnetometry equipment used in archaeological prospection; it detects subtle differences in soil magnetism that can indicate buried pits, ditches, hearths, or structural remains without any excavation. What the instrument found here was a sub-circular ring of negative magnetic gradient, roughly 1.5 metres in width and 8 metres in diameter, with what may be an entrance gap on the north-northeast side. A pit-type anomaly was also recorded within its eastern quadrant, and just outside the ring, two faint interconnecting linear features were detected, possibly the remains of slot-trenches that once held timber fencing or a palisade of some kind. The report is careful not to overstate the case; the language throughout is conditional, noting only that the readings "may denote" a stone structure. The site sits in close company with a stone circle 25 metres to the northwest and a burial mound 25 metres to the southwest, placing it within a cluster of prehistoric monuments at the northeastern edge of Lough Gur, one of the most archaeologically dense areas in Ireland.

There is, practically speaking, nothing to see here, and that is precisely what makes it worth knowing about. The field on Knockroe Hill is private pasture, and the structure itself has no physical presence above the soil. A visitor to the wider Lough Gur area, which lies around 680 metres to the west, will find the nearby stone circle and burial mound far more legible as monuments. But an awareness of what lies just beneath the grass nearby, a probable prehistoric structure known only through magnetic readings, unnamed and unexcavated, adds a different quality to the landscape, one of incompleteness and ongoing uncertainty that no interpretive panel can quite replicate.