"DADGAD" is an alternating tuning where the guitar is tuned to D,A,D,G,A,D. Many Irish guitar players prefer the sound of chords played with this tuning due to their more open, modal nature, giving greater freedom to the feel of the tune.
DADGAD allows players to accompany Irish music without dictating the sound according to the rules of a major or minor key. It instead creates a freer, more open sound, shaped by the melody of the tune itself.