IREX can be bowled over in a few weeks self study with the Tait book... If you have recently completed CPL subjects just brush up on some MET in particularly conditions conducive to icing etc.
Familiarise yourself with the AIP especially ENR 1.5,.. If you know where to look in the docs you'll have no trouble in the exam as it is not time critical. 3.5hrs for 40 questions is ample time.
Just work logically though the exam, I always found going through and bowling over the short questions first then followed by the questions that involved calculation and some thinking last worked for me! And if you get stuck just move onto the next question ! More often than not when you come back to it you kick into another gear and it becomes a straight forward answer!
Good Luck
