By the time John O'Donovan visited in 1837 and noted it for the Ordnance Survey, the place had already shifted its purpose. It was, he recorded, "formerly used as a common graveyard, but now only used for children who die prior to baptism." That phrase carries considerable weight. Children who died without being baptised were, under Catholic teaching of the period, denied burial in consecrated ground, and so communities across Ireland quietly set aside separate plots for them. These places are known as cillíní, and the one at Fassagh is a particularly layered example.

The Ordnance Survey's 1837 six-inch map shows the site as a roughly oblong enclosure, approximately 68 metres west-east by 40 metres north-south, marked with a perforated boundary line. By the time the revised 25-inch map was produced in 1910, it had grown considerably, recorded as a subrectangular enclosure of about 1.5 acres. When archaeologists described the monument in 1979, they found a roughly rectangular raised mound defined by an artificial scarp of earth and stone, though the scarp's edges were difficult to trace precisely. In the south-west quadrant there are irregular linear arrangements of set stones, and a low wall roughly 1.1 metres thick appears to extend southward. Large boulders occupy the centre of the site, and much of the interior is dense with tall grasses and weeds. Perhaps most intriguing is the reputed presence of a bullaun stone within the enclosure. A bullaun is a boulder or outcrop with one or more cup-shaped hollows ground into its surface, often associated with early Christian or pre-Christian ritual use across Ireland, and their appearance in burial contexts is not uncommon. Aerial photography shows the whole monument as a raised oblong earthwork sitting within a larger, irregular oval-shaped earthwork, hinting at a history that predates even its use as a graveyard.