When a road crew broke through the surface of a County Kildare lane in 1903, they found themselves looking down into a narrow stone passage that had been sealed beneath the ground, apparently for centuries. The tunnel, uncovered just north of the cemetery at Timolin, was barely sixty centimetres wide and less than a metre deep, its roof formed from flat stone flags laid across dry stone walls. It extended northward in the direction of a mill, and it was reported to have contained human bones.

Souterrains are underground stone-lined passages or chambers, typically associated with early medieval monastic or settlement sites in Ireland, and thought to have served as places of storage, refuge, or both. The Timolin example sat in the immediate vicinity of an early monastery, and its discovery was documented by Fitzgerald in the Journal of the Kildare Archaeological Society in 1903. A second souterrain, apparently similar in construction, was found around the same time close to the eastern end of the present church. Whether either passage was integral to the monastic complex, or predated or postdated it, was not established. The presence of human bones in the first tunnel adds a layer of ambiguity; souterrains occasionally intersect with or were later used near burial grounds, though the bones at Timolin were never formally analysed or explained in the surviving record.

Both souterrains have since been filled in, which means there is nothing visible at the site today. What remains is the knowledge that just beneath an ordinary road surface, and close to a churchyard that still receives visitors, the ground once held two carefully constructed stone corridors whose original purpose has never been fully resolved.