I was in the same situation as you a couple of years back, so maybe this will help. You also need to do some research into the Irish equivalent of LASORS in the UK, but I'm sure there will be a lot of similarities.
If you're looking purely at economy, then then doing the IR in a fixed wing SINGLE first will probably give you the largest bang for your bucks. There's a lot of just getting used to using the instruments and the procedures which you can do in the sim, or even on your PC with tools like RANT. You might as well do that phase in the cheapest possible vehicle. The overlap is reflected in the discounts on offer, which in the UK are 15 hours down from 55 for IR(A) conversions to helicopter.
Doing your IR before your CPL also gets you discounts on the CPL (in the UK) but they're not huge, and you're more likely to be driven by cashflow issues, ie. where's the pay cheque going to come from to fund all of this.
There aren't any cross dispensations between fixed wing and helicopter flying on the CPL side so that's more driven by the theoretical knowledge requirements.
AFAIK the ATPL transitional arrangements are still on offer in the UK whereby you can do mostly fixed wing papers and Helicopter PoF, adding the remaining Fixed Wing PoF and Performance later as required. That gives you ultimate flexibility (for 36 months) to do CPL and IR on either fixed wing and/or Helicopters.
The answer really lies in a more detailed analysis of where you want to end up, and how you intend to fund it all.
Hope that helps to get your thinking going, feel free to PM for further info.