The stone is the lower millstone of a horizontal water mill, the kind of grinding mechanism that powered rural food production across Ireland from early medieval times onward. In a horizontal mill, water was channelled to strike a wheel lying flat beneath the mill floor, turning a vertical axle that drove the upper stone directly, without the need for gearing. The lower stone, sometimes called the bedstone, remained fixed while the upper stone rotated above it. What survives at Tonlegee is that fixed lower stone, measuring 0.85 metres in diameter and 0.40 metres deep, with a hollowed surface and a centrally placed hole of 0.13 metres diameter through which the axle passed.

The stone was recovered from the grounds of St John's Church of the Evangelist in Tonlegee, in the old barony of Coolock, County Dublin. Along with the millstone, a few sherds of 13th and 14th-century pottery were found at the same location, which places the mill's likely period of use in the later medieval period. The association of milling sites with ecclesiastical land was common in medieval Ireland, as churches and monasteries often controlled local grain processing and took a portion of the yield in payment. Whether this particular mill was attached to a religious establishment or simply occupied land that later became a churchyard is not recorded, but the pairing of the millstone and the medieval pottery points to sustained activity on the site during that period. The find was noted by Swan in 1991 and compiled as part of the archaeological record by Geraldine Stout.

St John's Church of the Evangelist is in the Tonlegee area of Dublin, on the northern fringes of what is now the city's suburban spread. The millstone is not a dramatic landmark, and visitors should not expect the kind of preserved mill complex found at more prominent heritage sites. What is here is a single worked stone, significant for what it implies rather than what it displays. Those with an interest in the material culture of medieval Dublin will find the pairing of industrial and ceramic evidence a useful reminder that the landscape now covered by housing estates was once productive agricultural land, ground grain and all.