Slickerbill, one of the best out there is called "Irish Traditional Guitar Accompaniment" by Gavin Ralston. All of these books use alternate tunings. This one uses dropped D. In Irish "Trad" style tunes (as opposed to songs, which Alice is focused on), the rythm is is critically important. It is where the so called "Celtic Rock" bands blow it. Introducing new instruments into the music of our people is not a new idea, it has been going on over the whole history of the tunes. But there are some things that are characteristics of the music, and the unique 6/8 and 9/8 strums are part of that.

HERE> is the Elderly page on that book by Gavin Ralston. It is first class.

All the best,

Mick