What is DADGAD? It is an alternative to standard tuning (EADGBE) in which the notes become D A D G A D, from the lowest to the highest. This is what is called an open tuning because sounding all the open strings would make a chord: Dsus4. It is a widely-used tuning in Irish and Breton music and is sometimes used in rock and pop music.
DADGAD tuning is useful for playing traditional music because it maintains a "drone" note permanently. Also, chords that are difficult to achieve in standard tuning can be played, including the so called "modal" chord (no 3rd) and barre chords.
The first guitarists of traditional Irish music to use this tuning were Mícheál Ó Domhnaill and Dáithí Sproule in the 1970s.
Mícheál Ó Domhnaill & Kevin Burke (1982)