"Grinsell ('Folklore of prehistoric sites in Britain') says the cairn is the grave of Norse princess, or perhaps that of a giantess from Fingal's days."
Grinsell seems to mixing this cairn up with the other cairn on the summit of the other Beinn na Caillich where the other woman is buried. This one's the Norse princess, the other one's the giantess (Grainnhe herself, no less).