What makes it quietly peculiar is that it was probably never built as a bath house at all. Local tradition, recorded by schoolchildren in 1937, held that the early Protestant owners of the estate had dressed up an older Catholic oratory with a false front of honeycombed limestone blocks lifted from the river bed and smothered the whole thing in ivy, effectively hiding a holy well dedicated to St. John in plain sight. Once the ivy was eventually cleared, the structure was said to closely resemble the ruins of St. Mary's Church in the demesne graveyard, not far from the ruins of Sarsfield Castle. A blocked doorway in the nearby demesne wall at Tandy's Lane may once have allowed public access, though any memory of devotional rounds, or Stations, performed there has long since faded.

The spring itself came to wider attention in 1758, when it was noted on the demesne of Agmondisham Vesey, about a mile from Lucan House. A chalybeate spring, one containing dissolved iron salts, already existed nearby but had fallen out of use; the more notable discovery was sulphurous in character. The well measured almost 2.1 metres long, 0.6 metres broad, and roughly 38 centimetres deep, and it yielded a generous flow of water despite its modest dimensions. Its position on the bank of the Liffey meant that winter floods regularly swamped it, a problem Vesey addressed by building a protective wall. The waters attracted serious medical interest: they were reported to help with skin conditions, chronic rheumatism, paralysis, and various complaints including ringworm and impetigo, and the water was apparently sent to Steeven's Hospital in Dublin before 1766. John Rutty, who published a survey of Irish mineral waters in 1757, described the taste as resembling a boiled egg and, at its strongest, a putrid one, with a smell of sulphur. The spa remained fashionable until around 1845, when, as Daly put it, fashion withdrew her favour. The well may also be identified with Tobernaclugg, an older holy well in the same area that had long been associated with St. John, suggesting the site layered devotional and therapeutic uses over several centuries.

The structure is recorded on the Ordnance Survey maps as an oratory, aligned northeast to southwest within the demesne grounds. Lucan Demesne is a public park today, and the Bath House can be found close to the Liffey bank. An ancient small token was recovered there in the time of a Mr C. O'Connor, acknowledged by the National Museum in June 1934, which points to the site having functioned as a place of devotion well before the spa era. The surrounding demesne also contains the nearby Sarsfield monument mentioned by Lewis in 1837, which makes for a useful landmark when trying to orientate. Flood levels can still affect the low-lying ground close to the river, so the drier months tend to make for easier going underfoot.