The 60's were a rebirth for the acoustic guitar on both sides of the Atlantic. One of this « revival »'s most notable feature was the increasing use of Open Tunings.
In the US, C.S.&N. and Leo Kottke had their ears turned towards India and the Mississippi Delta; in England Davey Graham and Renbourn were busy creating what we know today as the Celtic Guitar and it's favorite tuning : the DADGAD.
The type of Celtic music we're familiar with takes us back to the XIVth century. However it's origins are even more ancient. It's rhythmic & melodic complexites are akin to Turkish or Central Asian musics.
Only recently has the guitar adjusted to this repertoire of over 2.000 traditional melodies. Celtic music was often played in bands, whether in Ireland, Scotland, Wales or Brittany, traditionally with a violin, accordion, harp, flutes, percussions & bagpipes. The Renaissance of Irish music has been simultaneous with the arrival of the guitar as used now as the banjo and even as the bouzouki whose Greek origin has been lost by many an Irishman. Naturally one can play flat-pick fiddle tunes, but the reason the celtic guitar has been so popular is because of it's Fingerstyle & Open tunings.
Open chords resonate longer & ampler, not unlike harp effects. If you're trying to get a long drone effect, a bass drone string is essential, which is given by the 6th Low D. As in many traditional musics, Celtic music depends very much on modal or fixed keys certain diatonic instruments can only provide, such as flutes, harps accordions or bagpipes. Thus, the most common keys are D & G.
There is a good reason for using the DADGAD, (pronounced dad-gad, & easy to remember), since the root is D, a good key for traditional tunes and 5th, 4th & octave intervals will insure maximum resonance. The second interval between G & A on strings 2 & 3 is handy since it avoids stretches. You'll find the 2nd interval in other celtic open tunings such as EADEAE (D & E) by Davey Graham one of it's pioneers or CGDGAD (G & A) by Dave Evans.
Pierre Bensusan is the one who has pushed DADGAD to the limits by using it as his basic tuning in a variety of musical environments and DADGAD has become his trademark. This Celtic tuning has gone beyond it's folk frontiers and has been used by all kinds of performers from Adrian Legg, Michael Hedges & Dan Ar Bras to Aerosmith's Joe Perry or Led Zeppelin's Jimmy Page in Kashmir and Black Mountain Side.
The resonance factor being the most prominent feature of the Open Tuning effect, playing must remain smooth by remaining perfectly synchronized between right and left hand and by maintaining a steady rhythm when playing triplets.