Others make the list precisely because nobody is quite sure what they are. A scrub-covered hollow in pastureland at Ballinvreena, County Limerick, sits firmly in the second category. It is recorded as a possible earthwork enclosure, but the surveyors who documented it were candid about the uncertainty: it may be nothing more than an old quarry hole, the kind of shallow depression that pockmarks agricultural land across Ireland without any particular story attached.

The feature first came to official attention through an oblique aerial photograph taken on 4 March 2006 as part of the Archaeological Survey of Ireland's aerial photography programme. From the air, it read as a roughly circular shaped enclosure, the kind of plan associated with ringforts or similar early medieval earthworks, which were typically circular banks and ditches enclosing a domestic or defensive space. Follow-up examination of Digital Globe orthoimagery captured between 2011 and 2013 showed the same roughly circular hollow on the ground. Significantly, the feature does not appear on the Ordnance Survey Ireland historic six-inch maps, the nineteenth-century surveys that recorded field boundaries, ruins, and earthworks across the country with considerable thoroughness. Its absence from those maps does not rule out antiquity, but it does remove one piece of supporting evidence. Compiled by Martin Fitzpatrick and uploaded to the national record in September 2021, the entry is a rare example of archaeology done honestly, noting what is visible without overstating what it means. A standing stone lies approximately 180 metres to the east, recorded separately, and its proximity is noted without any firm connection being drawn.

The earthwork sits in pasture roughly 130 metres west of a local stream, which gives some bearing on its location, though access to privately farmed land in this part of Limerick would require landowner permission. The scrub cover visible on satellite imagery means the hollow is not necessarily obvious at ground level, and without the aerial perspective that first flagged it, a visitor might walk past without registering anything unusual. The neighbouring standing stone, a single upright slab of the kind found scattered across the Irish landscape from prehistoric periods onward, is the more legible of the two features and might offer a more satisfying point of reference. For anyone with an interest in how archaeology actually works, though, the earthwork at Ballinvreena is instructive precisely because of its ambiguity: a circular shape, some scrub, a question mark, and an entry in the national monuments record that declines to pretend otherwise.