I've just arrived at this thread on the same quest as the originator, to know more about the song "I Lost My Love And I Care Not" which is so excellently sung in the 1967 film of "Far From The Madding Crowd". Much of what follows about the film is from memory, but I think it's correct ...

I agree with others about the use of genuine folk music in the film, which does much to give it an authenticity despite the rather lacklustre performances by many of the leading actors, the only one I'd unhesitatingly praise being Alan Bates as Gabriel Oak. The others all start off with an attempt at something like a West Country accent, but fail to sustain it beyond their opening scene, with the even worse exception of Terence Stamp who makes no attempt at a local accent at all, his accent sounding very much like far away London, and whose acting is rather wooden throughout, but particularly in the scene when as Troy he is shot by Boldwood. To cap it all, Bathsheba is described in the book as having dark or black hair, but in the film we have Julie Christie with auburn hair out of a bottle, and false eyelashes. For me, the film is saved by the wonderful use of the Dorset countryside, the excellent use of folk music, and the supporting cast who are without exception excellent, particularly Fiona Walker as Liddy. IIRC, some of them were genuine locals, and the guy who played Joseph Poorgrass had a steady living for some years afterwards advertising English butter and the like on TV.

The film was directed by John Schlesinger, who is very adept, as in his later 'Yanks', at conveying a sense of time and place, and the folk music definitely contributes to that. It was arranged for the film by the Scottish folk singer Isla Cameron, who sang "Bushes and Briars" (which Julie Christie mimed to) at the Harvest meal, which also featured the subject of this thread "I Lost My Love And I Care Not", and "The Bold Grenadier" over the scenes of Sergeant Troy wandering off and apparently drowning at sea - she also acted in the film, but for some reason ended up on the cutting room floor. Besides "I Lost My Love", other notable contributions are the use of the Dorset Four busked by Gabriel in the square after failing to get a job at the hiring fair, and Dave Swarbrick playing in the later Harvest Home which celebrates Bathsheba's and Troy's marriage, IIRC the wonderful tune played is called Davy Davy's Knick-Knack.

James Hogg, aka 'The Ettrick Shepherd' lived from 1770-1835, and, as already stated, 'Songs' was published in 1831, while Thomas Hardy's dates are 1840-1928, so Hogg had already died when Hardy was born, and thus they cannot have ever met let alone known each other. However, Hardy was obviously a literate and well-versed man, so I think it quite possible he might have known of Hogg's work without any further help, but if he did not there is a possible Scottish intermediary in William Strang 1859-1921, who was raised in Dumbarton but went to Art school in London, and was one of the foremost illustrators of his day - specifically he did some portraits of Thomas Hardy - as well as many other celebreties of his day, many of them from the world of literature - and illustrations for works by Robert Burns, a printing of Don Quixote, and many other works published at the time. So, if Hardy did not happen to be aware of Hogg's work already, I think it's highly likely that Strang would have known of it, and so may have mentioned it as the sitter and artist conversed during the making of the portrait - a good artist would wish to keep his subject interested as he worked, and, although at different ends of the country, Hogg's and Hardy's areas of interest were not dissimilar.

Of course, Hardy's song is different from that of Hogg's, but he could hardly have put a fairly obscure Scottish song in a work set in the West Country, so perhaps he merely anglicised and simplified it?