That mythological reputation has since been overtaken by something grimmer and more historical. The bones of at least forty-four people, the majority of them women and children, were found in the cave's innermost recesses, most likely the remains of those who fled into the dark to escape a Viking raid and were never able to leave.

The Annals of the Four Masters record that around 928 AD, a leader named Godfrey, grandson of Imhar, led the Vikings of Dublin in an assault on a place called Dearc Fearna, the Cave of the Alders, during which a thousand people were said to have been killed. The scholar O'Donovan, writing in 1854, cautiously identified Dearc Fearna with Dunmore, a case that later researchers found persuasive. Excavations in 1973 of the cave's two most inaccessible sections, the Market Cross Chamber at the southern end and the Rabbit Burrow to the north, produced human bones, glass beads, iron objects, half an amber ring, and a collection of silver coins. The coins, all recovered from the Market Cross Chamber, were struck no earlier than 879 and no later than 930 AD; four came from north-east England and three from York itself, all from the 920s. Since coins of this type were not in Irish use at the time, they were almost certainly carried in by the attackers and came to rest in the sediment by chance. The dating fits the broader chronicle record, which points to Guthfrith, a Viking leader expelled from York in 927, as a likely figure behind the Dublin raid. Earlier descriptions of the cave, including one by Molyneaux in 1709, had already noted skeletal remains, and isolated finds such as ringed pins, a decorated bronze strap, and an iron axe-head had long indicated that something violent had taken place here. The cave sits on a limestone ridge above the Dinin valley, with ringforts and the remains of an early church lying around four hundred metres to the west, the probable original targets of the raid. The cave was designated a National Monument in 1940 and opened to the public in the 1960s.