DADDAD tuning is one of the many D-based tuning used for guitar. It's most unique feature is lowering the third string from G note of DADGAD tuning all the way down to D. This gives users a pair of unison strings in the middle of the neck. It has been used in styles ranging from Irish folk to heavy rock. It is sometimes referred to as "Papa Papa" tuning.
DADDAD Tuning is used in Irish and Scottish folk music quite often, as well as for heavier alternative rock styles, many of which employ lowered tunings. It is used on Dobro guitar, as well, in rock and blues contexts. It has also been found in the hands of acoustic singer/songwriters.
DADDAD tuning demo.
Little information on the history of this tuning exists, other than that we know it originates in the folk music of Ireland and Scotland. It is the less popular sibling of DADGAD or Celtic Tuning, which is more widely used. Many different guitar tunings are found in these styles but DADDAD is one of the more obscure ones.
Stephen Stills, of Crosby, Stills, and Nash, is the most visible user of DADDAD Tuning in the modern era and many of his best-known songs are played in it. Other users include Australian folk-rocker John Butler, American New Age guitarist Billy McLaughlin, and Portuguese punk player João Silva.