At some point in the early Christian or medieval period, whoever laid out the graveyard at Knockeen simply built their boundary wall around an ancient megalithic tomb rather than remove it. The result is one of the more quietly arresting juxtapositions in County Waterford: a portal tomb, the type of Neolithic chamber grave commonly called a dolmen, sitting flush against a churchyard wall as though it had always belonged there.

The structure is complete and well-preserved, which is not something that can be said of every monument in this class. A portal tomb typically consists of two tall upright portal-stones at the entrance, flanking sidestones and a backstone that together form the chamber, with a large capstone laid across the top. At Knockeen, all of these elements survive: two portal-stones, two sidestones, a backstone, and a sillstone sealing the entrance threshold. A rectangular roofstone rests across the portal-stones, and a subsidiary roofstone covers the sidestones and backstone behind it. The whole structure faces north-west up a gentle slope, set into the south-facing hillside using local stone. George Victor Du Noyer, the geologist and antiquarian, documented the tomb in a paper published between 1864 and 1866, placing it within a broader study of cromlechs near Tramore. William Copeland Borlase included it in his three-volume survey of Irish dolmens published in 1897, and Seán Ó Nualláin returned to it in his 1983 analysis of the topography and distribution of Irish portal tombs, by which point it had accumulated a modest but consistent bibliography stretching back nearly a century.