I've had a look into where this marker came from. 

I think I probably added it from the IOM Government 1:25k outdoor map based on a 'Cairns' icon and label. It's likely this in turn came from either the 25 inch OS map (1870) [1] where there are a group of cairns marked as individual mounds, or the 10k OS map (1940s) [2] which marked individual mounds.

[1] https://maps.nls.uk/view/121146733
[2] https://maps.nls.uk/geo/explore/#zoom=16.0&lat=54.25735&lon=-4.44743&layers=193&b=1

It's likely they're mostly lost under the vegetation now, though now I've looked at the aerial imagery I think I can make them out. The path takes a slight detour around the edge of the middle one.

I've replaced the single marker with separate ones for each mound based on the 10k OS map, and tweaked them slightly based on the imagery.