Getting back to the original question -
I assume, from the wording of the original post "I am planning to go through the modular route for the ATPL after the PPL", that you do not yet hold a PPL and that, consequently, you are training for a JAR PPL. If that is the case, we can dispense with the idea of the IMC leading to exemptions from the IR.
"I believe these 150 hours need to follow some sort of pattern" - Not true. All that is required is 150 hours of flight time.
"Can these hours be used to achieve additional ratings on the PPL" - They can be used for anything, flying round in circles if you wish. It would clearly be more beneficial, however, to use them in a structured manner.
"Now, if a IR is taken at this stage does it count towards the IR after the CPL" - An IR is an IR, it doesn't matter what licence you hold or when you gain it.
"The point here is that 150 hours is a lot of money and probably it compensates to acquire extra ratings that would result in some sort of exemption later on." - If you hold an IR, the modular CPL course is decreased, by 10 hours of instrument training, to 15 hours. However, you should not underestimate the difficulty of the IR for an inexperienced pilot, nor the fact that you would have to pass the PPL/IR exams as well as the ATPL exams if you were to take this route. Gaining a night qualification and MEP rating during the 150 hours would save having to do them later, although you must have 70 hours PIC before commencing the latter.
"Finaly, if hour building is done outside the UK but in a JAA country what is necessary to ensure that these hours have credit" - Simply enter them correctly and accurately in your logbook. They can be done in any country (inside or outside the JAA) in any aeroplane (whether owned by a training organisation or private individual).