This is supposed to be the largest cave in Arran. When it was visited by the OS in 1977, they found “no visible indication of its prehistoric or recent religious use.” Perhaps they weren’t looking hard enough, because John McArthur said in 1861:
The Monster or Black Cave yawns beneath the bold cliffs of Benan Head [..] It has been used until lately as a place of worship by the Islanders. Within its walls the relics of ancient habitation have been discovered – arrow-heads, chipped and polished, and flakes of flint, mingled with the shells of the whelk and the limpet, indicating that here the native artist had his workshop and his kitchen, and wrought out from the rough pebble the frail weapons of the chase.
‘Antiquities of Arran’, p99.