The putative connection with the Armstrongs also figures, it seems, in Scott's Minstrelsy (I really must get my own copy). Looking in Christies's Traditional Ballad Airs just now —for something completely different— I came upon the set he gives of Gude Nicht, an' Joy be wi' Ye a', in that case with Lady Nairne's lyric, which I don't think we've had here yet. Christie (II 298-9) quotes Peter Buchan (Gleanings of Scarce Old Ballads —presumably— II 127):

"All that I have as yet been able to discover in print of this very old song, were eight lines, which have been quoted by Burns [ see above ], and many others since. Even the indefatigable Sir Walter Scott could discover no more for all his researches, and these he has given in the Minstrelsy of the Border, vol I, p 283. He conceives the lines to have been composed by one of the Armstrongs, executed for the murder of Sir John Carmichael of Edrom, warden of the middle marches: but I am inclined to think they have been written on another occasion, long prior to the time of Carmichael's death, which happened on the 16th of June 1600. The eight lines, alluded to, have long been current, and the air, to which they are sung, popular in Scotland. It gives me, then, particular pleasure to be able to lay this much admired relic, so often sought after in vain by the learned, in a complete state, before the lovers of ancient song."

Christie then quotes the text given by Buchan, which is much as that posted by Q: see above. Evidently there has been some confusion between this "last goodnight" and the —so far as we can tell— completely different Armstrong's Last Goodnight, dating back quite some time. It's likely enough, I'd think, that Margaret Irwin relied on Scott there, and thought he meant that the two were the same song.