The Pirra system (the land Pirra, the Pirrans, Pirran Pipe) is characterized by magic. To clarify where this question came from, I was wondering whether the magic of the pipe had an enduring effect, at least for a couple of hours. It is stated that the pipe was played in the morning, at noon and in the evening. This led me to theorize that between those specific times, shadows could still penetrate Pirra.
It is stated in Tales of Deltora that ordinary humans could not enter Pirra because 'powerful magic' barred their way. This could be the magic of the pipe but their are numerous reference in Deltora books that the Pirrans could wield magic as well. Without going into further detail, I have come up with a theory of how the pipe and magic worked.
As mentioned before, Pirrans could use their magic (not the pipe) to protect their land from ordinary humans. This was most likely done by some kind of shutting spell, much like the Torans. However, sorcerers were probably able to penetrate this spell and that is why the pipe was played.
It is obvious that while the pipe is played, all evil is suppressed and temporary powerless. I also believe that the Pirran Pipe somehow reinforced the magic of the Pirrans and Pirra itself. This sounds complicated. So, look at it this way: in my opinion their is some kind of connection between the magic, the land Pirra, the Pirran pipe (specifically its music) and the Pirrans. Obviously the Pirran Pipe plays a pivotal role in this system. However since the origin of the Pipe is unknown, it could be that the Pirran's had created the pipe themselves. Perhaps putting a vast amount of their personal magic in this object. Therefore it is hard to say which factor is specifically paramount with regard to the magic of the Pirra system.
This is all very theoretical but I hope this sparks some sort of discussion.