The way in is not quite where it should be: visitors descend through a depression in the ground, ringed by a low concrete wall and wrought-iron railings, into a hole that was never meant to be an entrance at all. It opened up during the nineteenth century when grave diggers accidentally broke through the roof of the chamber below, a fact noted by O'Hara and Westropp as far back as 1898.

A souterrain is an underground stone-built structure, typically associated with early medieval settlement in Ireland, used variously for storage, refuge, or concealment. The example at Killala is a particularly elaborate one. The accidental entrance drops into a subcircular chamber with corbelled drystone walls rising to a domed roof, a small recess cut into the western wall, and a lintelled opening at floor level leading east into a low passage. That passage, barely 0.8 metres high, connects through a series of tight creeps, the narrow crawl-through gaps between chambers, into three large rectangular rooms arranged roughly north to south. Each rectangular chamber is built with vertical walls and large stone corbels supporting massive roof slabs. The most quietly ingenious detail is a hidden chamber tucked directly beneath an alcove in the second rectangular room: what appears to be an ordinary shelf in the wall contains a trapdoor in its floor, dropping into a small concealed space, from which another creep leads onward. The whole network extends beneath the adjoining street to the east. A cross-slab, an early medieval carved stone, has been reused as a roof lintel in the main passage, which says something about the pragmatism of the builders, or perhaps about later modifications to the structure.

The souterrain, along with that reused cross-slab and the nearby round tower, constitutes the clearest surviving evidence for an early medieval ecclesiastical settlement at Killala. The site is prone to flooding from groundwater seepage, which has shaped its condition over time. A detailed survey and scale drawings completed in 1982 by archaeologist Noel Dunne, assisted by Kevin Gilvarry, provide the foundation for much of what is known about the structure's internal layout and dimensions.