Archaeological monitoring carried out at St. Mary's on Brookvale Road uncovered part of a sub-circular enclosure, a roughly rounded boundary feature approximately twelve metres in diameter, defined by a ditch around 1.2 metres wide and 0.3 metres deep. It is the kind of discovery that tends to surface only when ground is broken for construction or utility works, and then just as quickly disappears back into the record.

The find was documented by Mc Conway in 1996, and on the basis of its form and context it is considered to be medieval in date. Sub-circular enclosures of this kind appear across Ireland and are associated with a range of uses: ecclesiastical boundaries, domestic settlement, or small farmsteads. The dimensions here are modest, suggesting a relatively contained space, perhaps the boundary of a single farmstead or a small enclosed yard. Donnybrook itself, despite its urban familiarity today, had a significant medieval presence. It was the site of one of Ireland's most notorious annual fairs, and the broader area remained largely agricultural well into the post-medieval period, which makes the survival of features like this less surprising than it might first appear.

There is nothing to see at the site itself. The enclosure lies beneath developed ground on Brookvale Road, and no surface trace remains accessible to visitors. Its interest is entirely in what it implies: that the medieval landscape of Donnybrook extended further, and in more organised form, than the built environment now suggests. For anyone curious about the archaeology of Dublin's suburbs, the published monitoring report is the most direct way into the evidence, and the find sits within a broader pattern of medieval activity that county Dublin fieldwork has been gradually piecing together over recent decades.