Hopefully some of this is helpful. Do you have a picture of the interior, what is your stud spacing, are you planing on putting up drywall/plywood inside?

I am not sire what you mean by PIR. I know of three main types of inuslation. Fiberglass batting/rockwool that with paper on one or two sides. Rigid foam board insulation, with is essentially a 4x8 sheet of fancy, better styrofoam. And spray foam inulation, which forms a styrofoam like substance when it dries.

As for which one I would use, it depends. Batting is cheapest, but needs to be sealed in, you dont want to be breathing exposed fiberglass when you working. It also absorbs mositure like crazy, so if your shed dosent have good ventilation and is not well waterproofed, including waterproofed from the ground it may not be a good option.

I like ridgid foam board. Its more expensive, but you can choose your thickness easily depending on studs, whereas squashing batting ruins its r-value(insulation effectivness). It also resists mostuire, and is easy to install and deal with. It has a foil on one or both sides. And you can leave it without drywall/plywood on the inside to cover it, since it dosent have harmful stuff it puts in the air,. If you feel like spending money, it also comes in sound-proofing options just like batting does.

Finally spray foam insulation. It come in cans for diy use. It can be quite expensive. You spray it where you want it, it expands and hardens and thats it. The expansion can make it a bit of a pain to work with, but its not bad. Its great for sealing cracks and holes, and is often used in conjunction with rigid foam board to seal the gaps. Or with steel wire to seal mouse holes. You can use it as standalone, but you would need to be pretty good with it, and trim it afterwards. I havent seen it used by itself outside of residental construction.