That seems a bit bizarre.
This description only makes some sort of sense if the author wrote 'Port Blair on Loch Shieldaig' when he actually meant 'Port Lair on Loch Torridon'. But even then it's confusing.
There are two Loch Shieldaigs - one just a mile or two from Badachro in the parish of Gairloch, and one at the west end of Loch Torridon opposite Diabaig. He obviously can't have meant the one in Applecross, because you could only walk from Gairloch to there by going round the head of Loch Torridon.
The first edition of the Ordnance Survey map, surveyed in 1875, does not show a path south across the moorland from Badachro. See
https://maps.nls.uk/view/74428380. The obvious land route from Badachro to Diabaig is by the path that roughly follows the coast.
The Gairloch Loch Shieldaig is east-south-east, not south, of Badachro, and therefore in the opposite direction from the path to Diabaig if you are starting from Badachro.
There is a path south from the Gairloch Loch Shieldaig, but it peters out in the moorland south of Loch Gaineamhach.
Do any of the censuses describe anyone residing at Port Lair as a ferryman?
BTW I have found my copy of Dixon's 'Gairloch', published in 1886. The map shows the coastal path to Diabaig, but no path over the moor. It does not show Port Lair, or mention or indicate any ferry across Loch Torridon (though that isn't conclusive because that would be outside the parish of Gairloch, and the book concentrates on the parish).
It's quite likely that the author of the guide had never been to most of the places he was describing and therefore didn't really know what he was talking about.