There is, just outside Peel Castle a mound about 90 feet long known traditionally as the Giant's Grave [..] It may be of interest to note that the traditional giant of this grave is said to be the original of the three-legged Manxman, a legend which is suggestive of the many bodies found in these chambered tumuli, of which the legs are often found entire.
I do think Miss Buckland gets a bit carried away at times (but her urge to rationalise folklore is not unusual is it). From p350 in
The Monument Known as "King Orry's Grave", Compared with Tumuli in Gloucestershire
A. W. Buckland
The Journal of the Anthropological Institute of Great Britain and Ireland, Vol. 18. (1889), pp. 346-353.