A C harp played in "A minor" (i.e. in fourth position) is actually using A Aeolian mode. You could play Carolan's Welcome that way on a C harp. A G harp played in "E minor," fourth position again, is actually employing E Aeolian mode. You could, for example, play "The Butterfly" that way on a G harp. You can also use a D harp to play in E Dorian or B Aeolian. And a G harp could play tunes in A Dorian. Basically, third position plays Dorian modes and fourth position Aeolian modes. That sort of covers the "minor-sounding" tunes in Irish music, for example.