During construction work on the Four Courts Extension in 1984, excavators uncovered a series of arched foundations belonging to a structure that appears to predate the courts themselves by several centuries, its origins and original purpose still not fully resolved.

The foundations, stretching some 18.2 metres in length, were identified by excavator McMahon as likely belonging to a 16th-century building. What made them architecturally distinctive was their construction method: semi-circular arches, a form in which the curve describes a perfect half-circle, springing from solid masonry piers. The masonry throughout was heavily mortared and roughly coursed limestone, meaning the stone was laid in approximate horizontal layers rather than finely dressed and precisely fitted. This kind of construction speaks to a building of some substance, built to last, though whether it served a civic, religious, or domestic function remains unclear from what was recorded. The Four Courts complex itself occupies ground along the north bank of the Liffey that has seen continuous occupation and redevelopment since the medieval period, and buried remnants of earlier structures turning up during building work is not unusual for this part of the city, even if the specifics of any individual find can be difficult to interpret.

The foundations are not accessible to visitors, having been uncovered during construction and documented rather than preserved in situ. Anyone with a serious interest in the find would do best to consult McMahon's 1988 report, published in the relevant excavation records, which runs from pages 271 to 319 and provides the primary documentation. The Four Courts itself, designed by James Gandon in the late 18th century, is open to the public during legal terms, and the area around it rewards attention for the layers of history compressed into a relatively small stretch of riverfront ground.