Selloo almost certainly derives from Suidhe Lugha, meaning Lua's seat, where Lua is a shortened form of Molua, the sixth-century patron of the nearby parish of Drumsnat. That etymology alone places the site within a very early layer of Irish ecclesiastical geography, one in which a saint's name became so embedded in a landscape that it survived in the placename long after any structure above ground had disappeared.

St Molua was descended from the Corca Oiche sept of the Ulaidh and, according to a twelfth-century life written under Augustinian influence, was educated at the monastery of Bangor in Co. Down before moving to Drumsnat and eventually settling at Kyle in Co. Laois, where his principal church still stands. Selloo, though it falls within Clones parish and Monaghan barony, is thought to represent the church he established in Monaghan before moving on. At the centre of the site, a roughly rectangular burial ground occupies a grass-covered area of about twenty metres by fifteen. Bullaun stones, which are boulders or slabs with one or more rounded hollows worn or carved into them and often associated with early Christian sites, were recorded in the interior as recently as 1968, along with the crude base of a cross. The burial ground was believed locally to have been used exclusively for children after around 1800, though no grave-markers remain. When a drainage trench was cut across it in 1983, it turned up human bones and a fragment of a decorated high cross shaft, now held in the County Monaghan Museum. The burial ground sits within a much larger circular enclosure, roughly eighty to eighty-five metres in diameter, defined by a low scarp that is still clearly visible along parts of its arc. Later field banks approach from four directions and converge around the burial ground, overlaying the earlier ecclesiastical layout with the ordinary geometry of agricultural land use, leaving the two histories pressed together in the same ground.