A rath is a roughly circular earthwork enclosure, typically of early medieval date, used as a farmstead and defended by banks and ditches. Within the western half of this one at Doonflin, a flat-topped circular mound rises 2.7 metres from the ground, with a base diameter of 12.5 metres tapering to a top just 3.8 metres across. That proportionally narrow summit gives it a slightly formal, deliberate quality, more considered in its geometry than the simple heap of spoil it might first appear to be.

The mound is edged at its base by a kerb of large stones, and further stones protrude through the turf on its sides and summit, suggesting a structural core beneath the accumulated sod rather than bare earthen dumping. What the mound originally was, and when it was built, is not recorded with certainty, though its position inside a rath places it within a landscape of early medieval activity. What is recorded is its later cartographic life: Ordnance Survey mapmakers marked the spot as a trigonometrical station on their six-inch maps of both 1837 and 1913, meaning the mound's elevated, flat top made it a convenient fixed point from which to measure and triangulate the surrounding terrain. The same physical feature that made it notable in one era made it useful in another, and it sat patiently through both.