Dropped D tuning is used frequently in Irish music especially in flat picked rhythm playing. John Doyle, the guitarist in Solas and a highly regarded session player has made an instructional video that uses dropped D. I haven't seen it, but it often gets favorable mentions by players. Artie McGlynn, a pioneer in Irish guitar uses standard tuning, but still manages to create a very modal sound. Paul de Grae has written a book advocating DADEAE tuning. So while DADGAD is widely used in Irish music, it is hardly exclusive. John