It contains just a quarter of a mile of passages and at its deepest point, it descends to 150 ft (46 m), but it possesses some fine calcite formations.

[8] In the Annals of the Four Masters, dated to the 17th century, Dearc Fearna was recorded as the site of a great Viking massacre in 928 AD: "Godfrey Uí Ímair, with the foreigners of Ath Cliath, demolished and plundered Dearc Fearna, where one thousand persons were killed in this year as is stated in the quatrain: Nine hundred years without sorrow, twenty-eight, it has been proved, 'Since Christ came to our relief, to the plundering of Dearc-Fearna.

"[3] Gofraith, ua h-Iomhair, co n-Gallaibh Atha Cliath, do thoghail & do orgain Derce Fearna, airm in ro marbhadh míle do dhaoinibh an bhliadhain-si, amhail as-berar isin rann, Naoi c-céd bliadhain gan doghra, a h-ocht fichet non-dearbha, o do-luidh Criost dár c-cobhair co toghail Derce Ferna.While the human remains found in the cave are thought to be victims of the Viking massacre, this has not been reliably confirmed.

Many of the remains belong to women and children, and it is hypothesised that they are the bodies of people hiding in the cave who were unable to leave when the Vikings tried to smoke them out, dying from asphyxiation.

[9] In his reports, Foot meticulously documented his findings, and culled references from the writings of researchers over the preceding 120 years.