Not because it has been demolished or built over, but simply because it lies on a steep north-facing slope inside a coniferous forest, and from the ground it is entirely invisible. What we know of it comes almost entirely from a single cartographic moment: the 1945 edition of the Ordnance Survey six-inch map, where it appears as a circular enclosure roughly 55 metres in diameter.

Ringforts, broadly speaking, are enclosed farmsteads of the early medieval period, typically defined by one or more earthen banks and ditches. They are among the most common field monuments in Ireland, with tens of thousands recorded across the island. Most are found in open farmland where their circular outlines survive as grass-covered earthworks, visible from some distance. The Mountainthird example is something of an outlier. Its appearance on only one map edition suggests it may have been faint even then, detectable to a cartographer working at a particular moment before commercial forestry changed the landscape entirely. Whether the tree planting that now covers the site came before or after that 1945 survey, it has since swallowed whatever earthwork remained, leaving a monument that exists more confidently on paper than it does in the field.