I'll try. (Assuming MEIR)
You should always do your IR before CPL - because the CPL requires 200 hours. If you do the CPL first, then you'll still need to do an additional 17 hours afterwards, taking your total time to 217, plus 30 hours on the SIM. Don't get me wrong, simulators are useful, but are quite often the same price as an SEP, so you're better off training in a real plane. Why? Because it all counts towards your 200 hours which means you're only actually paying for the instructor.
10 hours is common to both IR and CPL, meaning you can do a 25 hour CPL followed by a 45 hour IR, or you can do a 55 hour IR (15 multi) followed by a 15 hour CPL (in a single). This is all assuming you do a 'full' IR course - there are three other options:
CBIR: requires 45 hours, can be done all in the aircraft. The last 15 must be Multi. A really good option: basically go through the IMC training twice, then 15 hours of IR training in a multi.
ICAO conversion: for guys with an ICAO PPL - get an IR as part of hour building - then convert. Requires 15 hours, minimum 5 multi.
ICAO CBIR conversion: if you hold an ICAO IR AND have 50 hours PIC under IFR, which you can get during your hour building, you can do a straight conversion. No minimum multi time required, maximum potential to save money.
If you want to get really creative, you can do any of the above in a single, then convert to multi, but that requires a second full IR test so I wouldn't recommend it. There are a lot of options, so do your homework!