In reality, the small rectangular structure now known as St. John's Oratory is an eighteenth-century bath house, its devotional identity largely a later invention layered over a more mundane, and rather malodorous, purpose. The building sits at the intersection of several overlapping histories, none of them quite what they first appear.

The spa at Lucan was discovered in 1758 on the lands of Agmondisham Vesey, about a mile from his residence. The well, roughly 2.1 metres long, 0.6 metres broad, and fifteen inches deep, sat so close to the Liffey that winter flooding regularly overwhelmed it until Vesey had a protective wall erected. The water was sulphurous, and John Rutty, who compiled a survey of Irish mineral waters, recorded that it tasted of a boiled egg and, at its strongest, of a putrid one. Despite this, the spa attracted considerable attention. Its waters were credited with relieving skin diseases, chronic rheumatism, paralytic limbs, leprosy, herpes, impetigo, and ringworm; the muddy sediment was considered especially useful. One account describes a man cured of loose teeth and sore gums simply by rinsing his mouth and drinking the water. The spa's reputation was substantial enough that its water was reportedly being supplied to Steeven's Hospital in Dublin before 1766. The bath house itself, which a contemporary account by Mrs Delaney placed next to Mrs Vesey's dairy, was thought even then to have originally been an oratory. A holy well called Tobernaclugg, associated with St. John, may also have occupied the same ground. The structural story goes further back still: the 1680 survey of the lands of Lucan refers to the old walls of a chapel on the same land parcel, and a 1547 entry in the Calendar of Patent and Close Rolls, recording the dissolution of the Augustinian house of Saint Thomas the Martyr near Dublin, lists among its properties a chapel described as midway to Lucan, which some researchers have associated with this site. The spa's popularity held until around 1845, when, as Daly drily observed, fashion withdrew her favour.

The site lies within Lucan Demesne, close to the river bank and some 365 metres southwest of Lucan House. The demesne is accessible as a public park, and the area near the Liffey, particularly in drier months when the ground is less liable to flood, offers the clearest approach to the general vicinity. The structure is recorded on older Ordnance Survey editions under the label oratory, which gives some sense of how thoroughly its spa origins were forgotten. The holy well associated with it, Tobernaclugg, is catalogued separately in the national monuments record. Visitors interested in the layers of the site should look for the proximity of the river and the wall Vesey had built, details that make more sense once you know the water was the point all along.