This appeared to be a good example of a crannog, albeit a fair distance away from the wooded picnic area at the western end of Loch Achilty. According to the Highland HER (MHG7791) it is:
'...a tree-covered, artificial island about 80 yards from south shore of Loch Achilty. It measures 54' - 60' by 34' - 42' and stands about 2' above normal water level. It is built of evenly coursed stones, and has a regular batter, foundations measuring 79' by 59'. Timber projects from the NW, SW and east sides. [O Blundell 1913; D MacDonald, A Polson and D Brown 1931].'
The other island within the loch, Eilean Mhielidh (NH 43485702) is 'undoubtedly natural and no trace of buildings could be found on it [OS (R D) 12 July 1965]'. It is apparently not clear whether this island or the crannog is associated with a gentlemen known as MacLea Mor.
NOTE: don't forget to check out the nearby henge Achilty.