When a quarry face was cut back near Ballygambon in 1906, the exposed rock revealed something unexpected: the entrance to a cruciform limestone cave, its chambers arranged in the shape of a cross and extending into a northeast-to-southwest fissure deep within the earth. Along with the cave's unusual geometry came an equally unusual collection of finds, metal objects, stone and amber ornaments, large quantities of animal bones from sheep, pig, deer, and rabbit, and scattered human remains. The place became known as Brothers' Cave, a name whose origin is not explained by the surviving record but which has a certain subterranean gravity to it.

The exploration was led by Surgeon Lieutenant Colonel R. W. Forsayeth, whose report was published in the Journal of the Royal Society of Antiquaries of Ireland in 1931, some years after his death. The cave was entered initially through what was described as a secondary entrance in the quarry face at the western end, but a second opening was later made at the northeastern passage, at the north end of the north transept, and this became the focus of most of the digging. The accumulated material pointed to occupation that was probably medieval in date, though some artefacts appeared to belong to an altogether earlier period, possibly the Neolithic, the broad prehistoric era spanning roughly 4000 to 2500 BC in Irish terms. A further complication lies roughly seventy metres to the northeast, where the entrance to a separate cave called Oonaglour is located. Investigators suspected a connection between the two systems but were unable, or chose not, to reopen it. The original field notebooks from the excavation were for a time lost and later recovered, as reported by Corlett and Dowd in 2002, meaning that a fuller picture of what Forsayeth and his colleagues found in those limestone chambers may yet emerge.