Workers cutting into the face of a small hillock beside the Mullingar to Kells road sliced through the entrance passage of a structure that had lain undisturbed beside a busy route for centuries. The accidental exposure was recorded the same year in the Journal of the Royal Society of Antiquaries of Ireland by Murray, whose published plan remains the primary document of what was found.

A souterrain is an underground stone-built passage and chamber, typically associated with early medieval ringfort settlements in Ireland, and most likely used for food storage, refuge, or both. The Reynella example follows a recognisable layout: a short north-south entrance passage leads into a longer passage running roughly east to west, which in turn opens into a rectangular corbelled chamber at its eastern end. Corbelled construction means the walls are built with stones that progressively overhang one another until they meet at the top, forming a roof without mortar or a true arch. The chamber measures 3.8 metres north to south, 2.3 metres east to west, and stands 2.3 metres high, making it a substantial space by any measure. The hillock into which it is set is itself modest, roughly 24.7 metres by 18 metres, tucked into the northwest face of the rise just southeast of where a concrete roadside wall now stands. Two ringforts lie within easy distance, one roughly 280 metres to the northeast and another about 320 metres to the east, suggesting this corner of County Westmeath was a settled and organised landscape when the souterrain was in use. The road at this point also marks the old townland boundary between Balrath North and Crosserdree, a reminder that administrative lines sometimes preserve the outlines of much older divisions.