"Burt's Letters from the North of Scotland" published 1754 though written 1726.
I'm reading a reprint of this work at the moment, and something I read had me puzzled -

"The Irish Tongue was, I may say lately, universal in many Parts of the Lowlands; and I have heard it from several in Edinburgh, that, before the Union, it was the Language of the Shire of Fife, although that County be separated from the Capital only by the Frith [sic] of Forth, an Arm of the Sea, which from thence is but seven Miles over; and as proof they told me, after that Event (the Union) it be became one Condition of an Indenture, when a Youth of either Sex was to be bound on the Edinburgh side of the Water, that the Apprentice should be taught the English Tongue."

The political Union of England and Scotland took place in 1707. I find it hard to believe that the population of the whole of Fife, or even much of Fife was Gaelic speaking at that time. There may have been some Gaelic speakers in remote districts, but that is all I'm willing to accept until presented with further evidence.