Gruagach, a supernatural female who presided over cattle and took a kindly interest in all that pertained to them. In return a libation of milk was made to her when the women milked the cows in the evening. If the oblation were neglected, the cattle, notwithstanding all precautions, were found broken loose and in the corn; and if still omitted, the best cow in the fold was found dead in the morning. The offering was poured on 'clach na gruagaich,' the 'gruagach' stone. There is hardly a district in the Highlands which does not possess a 'leac gruagaich' - a 'gruagach' flagstone [...]
The following account was given to me by a woman at West Bennan in Arran in August 1895:
The 'gruagach' lived at East Bennan in a cave which is still called 'uamh na gruagaich' - cave of the 'gruagach', and 'uamh na beiste' - cave of the monster. She herded the cattle of the townland of Bennan, and no spring-loss, no death-loss, no mishap, no murrain, ever befell them, while they throve and fattened and multiplied right well.
The 'gruagach' would come forth with the radiant sun, her golden hair streaming on the morning breeze, and her rich voice filling the air with melody. She would wait on a grassy hillock afar off till the people would bring out their 'creatairean,' creatures, crooning a lullaby the while, and striding to and fro [...*]
The people of Bennan were so pleased with the tender care the 'gruagach' took of their corn and cattle that they resolved to give her a linen garment to clothe her body and down sandals to cover her feet. They placed these on a knoll near the 'gruagach' and watched from afar. But instead of being grateful she was offended, and resented their intrusion so much that she determined to leave the district. She placed her left foot on Ben Bhuidhe in Arran and her right foot on 'Allasan,' Ailsa Craig, making this her stepping-stone to cross to the mainland of Scotland or to Ireland. While the 'gruagach' was in the act of moving her left foot, a three-masted ship passed beneath, the mainmast of which struck her in the thigh and overturned her into the sea. The people of Bennan mourned the 'gruagach' long and loudly, and bewailed their own officiousness.
*There's a song here. But as it's long and Mr Carmichael was allegedly quite Creative when it came to Tradition, I'm leaving it out. You can see it on google books though in v1/2 of 'Carmina Gadelica' by Alexander Carmichael (1900). The huffy behaviour after being given clothing as a present sounds reminiscent of the behaviour of a Hob-Thrush.
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.