Whilst some historians have suggested traces of a circular enclosure once existed here, no visible evidence remains today. The modern church within the graveyard stands just north of where an older church once stood; a substantial building that measured 52 feet by 21 feet with walls three feet thick and reaching up to 14 feet in height before it was demolished in 1828. According to historical records, this earlier church featured a distinctive vaulted, arched tower at its western end.

Just south of the present church, visitors can find the modern entrance to a fascinating souterrain that showcases the complex engineering skills of early medieval Ireland. The entrance leads down through a subcircular shaft, nearly two metres deep, which narrows into a rectangular opening at the bottom. From here, stone steps drop into a central chamber measuring 5.6 metres east to west and 2.3 metres north to south, with partially corbelled walls that narrow towards the top before being capped with carefully placed slabstones. This central space connects to two additional chambers; an eastern chamber stretching over six metres in length, and a western chamber extending an impressive 8.7 metres. The construction throughout demonstrates sophisticated drystone techniques, with the base walls formed from large, thin slabs set upright.

Of particular archaeological interest is a re-used cross slab incorporated into the roof of the eastern chamber, bearing an incised circular depression with a groove and straight stem, though much of the design remains obscured. The western chamber shows evidence of what may have been the original entrance, with roofstones stepping upwards towards its end. Throughout the complex, the builders employed clever engineering solutions, including H-formation support stones and carefully fitted smaller slabs to fill gaps between the main roofstones. This remarkable underground structure occupies the most prominent position on the Glencolumbkille valley floor, surrounded by good pasture land, testament to its importance in the landscape of early Christian Donegal.