The structure known locally by some as "Sunday's Well" is thought by folklorist Caoimhín Ó Danachair, who documented it in 1955, to be the entrance to a souterrain, an underground passage or chamber typically associated with early medieval settlement in Ireland, often used for storage or refuge. It holds water, which is presumably what has long encouraged the classification, but the distinction matters: this is likely a man-made stone-lined tunnel whose mouth has, over centuries, been quietly mistaken for something else entirely.

Ó Danachair recorded the site under the name Toberreendoney, which appears on the Ordnance Survey map of 1840, the name suggesting an older Irish designation for a well associated with a particular person or place. What makes the site stranger still is its relationship to a second feature nearby. Just forty-five yards away, in the same field, stands what is recorded as Lady's Well, a site with its own separate entry in the monuments record. The Thomastown feature is said by local tradition to mark the original location of that Lady's Well, suggesting the devotional focus may have shifted at some point across the field, leaving this earlier spot without ceremony or continuing practice. Ó Danachair noted that, despite the name "Sunday's Well" surviving in some local use, there is no tradition of religious devotion attached to it.

The site sits in private farmland, so access would require local enquiry. The two features, the souterrain entrance and the nearby Lady's Well, are close enough that a visitor with permission might reasonably look at both in a single visit. The souterrain opening itself is worth examining carefully; such structures are found across Ireland and their entrances can appear deceptively modest, sometimes little more than a low opening between stone flags. The water that collects inside is incidental rather than sacred, though the centuries of misidentification suggest it has done a convincing enough impression of the real thing.