That thread was  Haunted by a song.  As I said there, Burns' song (1791) was originally set to Rory Dall's Port.  This melody was attributed to Ruaidhrí (Rory) Dall Ó Catháin, an Irish harper resident in Scotland from around 1603, though some authorities consider that it was in fact composed by James Oswald; at any rate, Oswald gave the tune its title when he published it around 1756.  To confuse matters, there are three tunes of this name, which may or may not be related.

When John MacLean of Tiree wrote Hi Oro 'S Na Horo Eile, he set it to an existing tune, which is attributed to Roderick (Rory) Dall Morison, a completely different blind harper, who was a Scot and lived from 1656 to c.1714; he was harper to MacLeod of Dunvegan.  The attribution may or may not be accurate; offhand I can't find a reference to it in William Matheson's book about Morison (The Blind Harper, 1970), but then, the book lacks a proper index.

Both tunes easily pre-date Burns' song, of course, but so far as I know it was Keith Norman MacDonald (in The Gesto Collection of Highland Music, 1895) who set Burns' text to Morison's (unnamed) melody, to which it is now commonly sung.  Perhaps he confused tunes, or harpers; or perhaps the connection had already been made by somebody else; there is a certain similarity between the tunes, though I don't know if they are actually related.  At all events, there need be no confusion in your mind over dates.  I gave links to examples of both tunes in your original thread on the subject.