"a folk song is one that folk sing!"
A far to general statement to be much use, in my opinion.
Lady Nairn's songs fall into the category of 'popular poetry' alongside Cunningham, and Burtns, rather than creations of 'the folk' - 'the voice of the people', which is what I believe folk songs are.
There's an excellent detailed discussion of Lady Nairn's songs, including, 'Land o' the Leal' in Alfred M William's, FOLK SONGS and POPULAR POETRY - well worth a look at
Jim Carroll