The IREX teaches you next to nothing about how to fly the actual approaches, it's mainly theoretical stuff like mcgrath mentioned.
Things like knowing all the particulars around recency and alternate requirements is a major part of the IREX and is not straight forward (but what about Australian air law is!?)
Worry about the flying portion once you pass the IREX.
Plus I doubt any of the FAA texts would have anything about NDB's
