A crannog is an artificial or partially artificial island, typically constructed during the early medieval period as a defensible dwelling place, built up from layers of timber, peat, brushwood, and stone in shallow water. The one in Tirnahinch Lough is known to exist because objects were recovered from it, but the site itself has never been precisely pinned down.

The lake, Tirnahinch Lough, measures roughly 220 metres on its northwest to southeast axis and around 180 metres across at its southeastern end. Its eastern foreshore is marshy and wide, extending about 50 metres inward from the water's edge, and the southwestern shore is broader still, around 100 metres of swampy ground before firmer land begins. It is somewhere within those waterlogged margins that the crannog is thought to lie. What is certain is that a Dr S. A. D'Arcy retrieved artefacts from the site during the late nineteenth or early twentieth century, and those objects are now held at Monaghan County Museum. Beyond that, the crannog remains unlocated, its outline dissolved into the reed beds and soft ground that fringe the lake on two sides.