The only way to get really at home with modes in folk music is to get yourself acquainted with the SOUND of the common ones, then you can forget about the theory.
Then you will know, for example, that Drunken Sailor is Dorian and will fit in E on a D-whistle without having to work it out.
There are NO rules to harmonising or accompanying modes - by definition they were melodic. Just play what sounds best. No standard set of chords will be completely within any of the minor modes, but just use discretion and you'll be OK.