Scotland's 'mainland island' would in fact be an island if it were not for the mile-long isthmus between West Loch Tarbert East Loch Tarbert.  Indeed, in the eleventh century, when the Scottish King, Malcolm Canmore, allowed Magnus Barefoot, King of Norway, to lay claim to any island he could circumnavigate by boat, Magnus succeeded in dragging his boat across the Tarbert isthmus and added the peninsula to his Hebridean kingdom!!