Traditional Irish Music
The Ionian is the most common and sounds "major" in all respects. The Mixolydian sounds almost the same as the Ionian, but includes a "minor 7th". The Dorian and Aeolian Modes both have "minor 3rds" but differ. The Dorian has a "major 6th" along with the "minor 7th", while the Aeolian has a "minor 6th" along with its "minor 7th".
Examples of the 4 modes:
Ionian - Key of D (Major) "Haste to the Wedding"
Mixolydian - Key of D (Major/Minor Mixed) "The Connachtmans Rambles"
Dorian - Key of A (Minor) "The Cliffs of Moher"
Aeolian -Key of B (Minor) "Tommy Peoples"
Another trait of some Traditional tunes is to include "half-sharp" notes, which are "quarter tones" halfway between the natural and sharp notes. They usually occur on the 3rd or the 6th of the scale.
Traditional Irish Music Structure
Traditional Irish Music Time and Tempo