The mound near Ballynacarrigy, some 210 metres south-east of Rock Brook House, measures 17 metres in diameter and is defined by a scarp edge with a shallow external fosse, the term for a ditch running around the outside of an earthwork. Traces of cultivation ridges survive both on the mound itself and in the surrounding field, hinting at the various ways the land has been worked and reworked over the centuries.

When the Ordnance Survey mapped this part of Westmeath at six-inch scale in 1837, the mound was recorded as a circular earthwork but was not flagged as an antiquity, which tells us something about how it was understood at the time. A similar earthwork lies roughly 110 metres to the north, suggesting the two features may share a common origin. The leading interpretation is that the mound began life as a barrow, a burial mound of the kind raised across Ireland during the Bronze Age or earlier, but that at some point after 1700 it was remodelled as a tree ring, the low circular platform planted with trees that was a characteristic ornamental feature of eighteenth-century demesne landscapes. Rock Brook House, sitting close by, would have provided both the motive and the means for that kind of embellishment. The result is a monument that has been quietly obscured by its own repurposing, its prehistoric origins dressed up in the aesthetic language of a later era.