Donald,
I came across a little more information about Donald Mackay of Clasneach, hiding in the genealogy of the Sandwood Mackays.
The Hon. Charles Mackay, son of Donald 1st Lord Reay by his 3rd wife, had 3 sons and 2 daughters, the younger daughter being Margaret. She married John Sutherland of Keoldale and they had 2 children, (1) Kenneth Sutherland [who, as previously mentioned, became the manager of Lord Reay's Durness barony], and (2) Barbara Sutherland, who married Donald Mackay of Clasneach. They had 3 children: George [already mentioned]; Marion who married John Mackay, Borley; and Mary who married Neil Morrison, Kylestrome.
I took another look at your earlier post and wonder where you found the approx. marriage date of 1749 for John, Borley and Marion Mackay? This seems a bit early for two reasons: (1) I have not seen Marion's age recorded anywhere but allowing for the generations it seems compressed; (2) their first child was apparently born in 1766*, i.e. 17 years after they were married.
* unless some were born prior to the commencement of the Durness Parish Register in 1764
BTW, as you probably know, "John of Borley" indicates that he has some legal title to the land. "John in Borley", or "John, Borley" indicates he rents it.
The patronymic in Mary's baptismal registration is helpful but, unfortunately, the Book of Mackay does not list any "issue" for George., nor does "The History of the House and Clan of Mackay".
Ian