Nigel Callaghan has some folklore about the area on his Taliesin website . He says that the Christian and prehistoric remains here are associated largely with Columb Cille (St. Columba), who's said to have preached here in the 6th century.
"The turas is a pilgrimage performed annually on 9th June. Starting at midnight pilgrims (ideally bare-footed) walk round the fifteen stations of the turas, saying various prayers as they go. Whilst some of the stations are directly associated with Columb Cille (like his chapel), many of them are pre-Christian standing stones and tombs, which were 'adapted'. This example is a standing stone, which has had beautiful celtic crosses inscribed on it."
He mentions the well of St Columba which is surrounded by a massive cairn, allegedly "built from stones carried up by the pilgrims on the turas, who take a drink from the well before continuing on their journey.
In the ruined chapel there is the saint's bed (a stone slab) - the clay beneath it has healing properties. There's a wishing stone nearby, and also St Columba's chair.
The church with the souterrain is just north of the earlier chapel.
There's a grainy, two tone image in a local guide to Columbkille that shows what really looks like a cup with a single ring and duct. The text implies it's a cross, but it really doesn't look like one. The slab is part of the roof of the souterrain which runs below the churchyard.