This is a fascinating thread. I have played pennywhistle, or tinwhistle, for about 25 yrs. While whistles come in several keys, if you half cover some holes, or "half-hole" them you can play in virtually any key on any whistle. Try running scales of half and whole covered holes, or sliding fingers off the holes and back, and you will hear what I mean.
    While I do read music, I learned pennywhistle entirely by ear. Perhaps that is why I find it easy to follow many different styles of music on the whistle.
    Getting the high notes and the octave transitions to sound right is more a matter of breath and finger control than anything else. I usually manage to make reasonable music on anything with six holes and a fipple. (recorders drive me nuts!)
    Someone mentioned having trouble with the octave transitions. All I can say is "Keep practicing!" I still have trouble with some pieces in that area. Some day I want to play "The Black Nag" fast enough for the dancers!

Ingrid