Millin Bay Cairn is a Megolithic Burial Chamber.
This is not a stone circle but a neolithic burial site which has been covered over (leaving the tops of the tallest stones visible) giving it the apperance of a stone circle.
When the site was excavated in 1953 they found at least 15 individuals were buried on the site.
The site has been back filled to protect it.
Another one of my "Haunts of Ancient Peace" despite there being a house right beside it!
The Cache is a disguised Micro.
It is NOT on the site.
It is hidden in plain sight and shouldn't be too hard to find.
As usual, you won't need to manhandle it too much and I'm sure you will return it to its wee home.