According to Ewan MacColl in his Folk Songs and Ballads of Scotland, "James Hogg in his 'Jacobite Relics' places this song as belonging to one of the Jacobite risings, either in 1715, or 1745. MacGillavry of Drumglass is one of the chiefs mentioned in the Chevalier's Muster Roll of 1715, and in the Forty-Five rebellion the powerful clan MacIntosh was led by a Colonel MacGillavry. A bard belonging to this clan may well have written the song; on the other hand, the name might have been used as a convenient designation for loyal highlanders."
The chords to Donald Macgillavry as given by MacColl are: Em, D, Em, Am, G, D (twice). As Susan of DT points out the words are available in the database. Can't help on Isla Water.