The barrow in Longstone, County Limerick does the opposite. Its interior sits below the surrounding ground level, forming a waterlogged, pond-like hollow enclosed by a broad circular bank, which is precisely what makes it unusual. A pond barrow, as this type is known, inverts the usual logic of the form, creating a sunken space rather than a raised one. They are rare in Ireland, and the example at Longstone is quietly anomalous even within its own category, sitting in ordinary improved pasture with little to announce its age or purpose.

The monument appears on the 1897 edition of the Ordnance Survey 25-inch map as a subcircular earthwork, already old enough by then to be simply a feature of the field system. A detailed description by O'Dwyer in 1959 recorded a circular earthwork roughly nine metres in diameter, enclosed by a bank with a basal width of around 7.3 metres, a gently sloping outer face, and an almost vertical inner face rising to about three metres. A gap of roughly 1.8 metres in the bank on the eastern side may represent the original entrance. When the Archaeological Survey of Ireland revisited the site in 2008, they recorded a bank still approximately 1.6 metres high on the exterior, though noticeably lower on the south-south-east to south-south-west arc, where cattle have worn a path across it. A report from Sarah McCutcheon, Limerick County Archaeologist, had already flagged damage in 2001, noting that the bank had been reduced in height at the south-east, its outer slope straightened along the northern to southern arc, and material removed from the base, exposing the underlying structure.

The barrow sits around thirty metres west of a small stream that also marks the townland boundary with Boherroe, a useful locating detail if you are navigating on foot across what is otherwise undifferentiated grazing land. Aerial and satellite imagery taken between 2005 and 2018 shows it as a roughly oval, tree-planted earthwork, which makes it marginally easier to spot from a distance than its modest profile might suggest at ground level. The interior edges are covered in dense overgrowth and the hollow itself remains waterlogged, so close inspection is awkward. The bank on the southern side, where the cattle path has done its work, gives the clearest sense of the cross-section, though it is also where the original form has been most compromised.