FutureFO, for what its worth, I did the IREX exam nearly 30 years ago so my perspective may be a little historical, but I also found it to be relatively easy - second only to the BGT exam. That said, I did it in the middle of my IR training - so was a combination of self-study and briefings as part of the IR training on use of navaids, minimum equipment, alternate requirements etc. Seems to me if you can fly IFR competently and know the rules under which you operate then you should be able to handle the exam.
Doing the IREX along with your IR training puts the knowledge in the context of practical operations and enhances understanding.
For me, the hardest exam for CPL/MEIR was MORSE CODE !!!
And naturally, not long after I passed the morse test they dropped the requirement!
Man I hated that thing - but 30 yrs later I still don't need to look up the cheat sheet!
Dr 