According to Gabriel Beranger's records, the already fragile ruins of Clondalkin's medieval parish church collapsed in 1787 as a direct result of the explosion at Caldbeck's Powder Mills. What had survived Vikings, the reorganisation of the Irish church, and the long indignity of abandonment could not withstand the percussion of a gunpowder factory going up. Today, all that physically remains of a building that once measured roughly 35 metres east to west and 16 metres north to south is a single chunk of masonry, about 2.8 metres long, standing in the south-east corner of the graveyard of St John's Anglican Church, which was built across the western end of the medieval structure around 1785 to 1789.

The site's history reaches back to the seventh century, when St Mochua (also known as Cronan) founded a monastery here at a place whose Irish name, Chluain Dolcáin, means Dolcán's Meadow. The monastery was significant enough to be plundered by Vikings in 833, after which Clondalkin briefly served as a base for Viking operations in the wider area. The relics of Mochua himself had been formally translated to the site in 789. Like the Célí Dé monasteries at Tallaght and Finglas, Clondalkin's lands passed into the see of Dublin before the Anglo-Norman invasion, a process connected to the ecclesiastical reforms that followed the Synods of Rath Breasail in 1111 and Kells in 1152. Under the Norman archbishops it became the administrative centre of one of the largest manors attached to the metropolitan see, and Archbishop Henry of London later assigned it to the dean of St Patrick's Cathedral. A will made by William Neill in 1471 records that the medieval church contained three altars, dedicated to Saints Mary, Thomas, and Brigid. The church began its long decline after the mid-seventeenth century, when its parish was united with that of Tallaght, though part of it was restored in the early eighteenth century before the powder mill explosion ended matters definitively.

What makes the surviving fragment especially worth examining closely is a memorial plaque set into the external face of the south-east angle of the east wall. Visible in Thomas Archdeacon's drawing of July 1791, the plaque carries a Christogram above an inscription that reads: "This stone was erected by Laurence Ternan. Beneath lies interred ye body of his father Patrick Ternan who departed 25th July 1754." The plaque has remained in place through everything. Earlier drawn records of the church also survive in the National Library of Ireland, including Gabriel Beranger's 1767 depiction and Archdeacon's 1791 view from the south-east, both showing the ruins in their final decades. Beranger's image may itself draw on a survey made by Samuel Molyneux around 1709. The graveyard at St John's in Clondalkin is accessible, and the remnant masonry stands quietly in its south-east corner, easy to overlook without knowing what it once was part of.