Leeneia, it means a hotchpotch , a hybrid, as in Jeannie Robertson's version which has one verse of McWilliams and one verse of Tannahill, and some of the Ballamena versions in Grieg Duncan.
As far as chords go, if it is played in D major, then just switching between D major and E minor (G's relative) works. Where to use them should be obvious to a guitar/singer using his/her ear.