This is a finger-style arrangement of an O'Carolan tune.
The tune was written in G.
Tony's arrangement is in open D, with strings tuned as follows (from lowest sounding to highest): D A D F# A D.
The booklet contains both the finger-style information, and a traditional version if you're not quite ready to tackle that!
If you're learning the Open D version, Tony suggests learning a couple of bars at a time. It's worth the effort.