Hi Toby, no problem.
150mm of PUR is just about minimum here to meet code.
You'll find that you'll probably end up in the same place in terms of minimum R value as us eventually, just takes years for things to change.
Also another point. If you ever intend to run the underfloor heating from a heat pump you want the tightest possible pipe spacing throughout - generally 100mm.
This improves the thermal transfer to the slab allowing you to run lower flow temperatures, which in turn increases the COP of your HP.

Let's all recall that we are in vastly different climates with code (or rational engineering) being different in Ireland vs Oklahoma where I live.

In general, the further north toward the respective pole (north or south) you go, means that a thicker underconcrete foam layer is needed and has a quick return on investment.

In Oklahoma, our deep soil temp, which become the underslap temp, is about 60 F, whereas Ireland is likely some 20 degrees less.

Even here in Oklahoma, I would put in some subslab insulation, especially if I do the labor (which is minimal. But maybe only 1 inch - ~ 25 mm. But guess, what - code here does not even address subslab insulation.

I tip my hat to the Irish, with their colder soil temperatures, for insisting (code) on at least 150 mm subslab insulation. Not only does this save money, but the slab floor is just SO much more comfortable!


Steve

I'm building a new house in climate zone 5 dry.

Average air temperature here is 41*F (5*C) so that is also the 10' deep soil average temperature.

I'm using 6" of EPS type 2 under my radiant slab and running down the inside of my foundation stem walls.

180' 1/2" pex-A loops 7” OC using constant circulation with Grundfos Alpha pumps on low, with Erie BB3000 injection Outdoor Reset controller.

Thermostats are Tekmar PWM type, that learn, with slab sensors.

You also need spray for foam insulation,its mechanical systems that heat and cool buildings are continuously operating; reducing extreme temperature variations saves on the overuse of mechanical systems and leads to lower energy bills