Pressure regulators don't give any reading, that's done by pressure gauges. Those can (rather rarely, though with cheaply made ones perhaps more often) go bad, as can the regulators (which typically cost under $100 for household size if you swap it yourself.) Even cheaper and less plumbing required is rebuilding the regulator in place, by replacing the internal parts only.
It's not clear (as you've given few details) how this pressure regulator (aka pressure reducing valve - same thing) relates to your irrigation. In most cases irrigation has a further reduction in pressure from household pressure. Irrigation typically is a very high flow rate load, and it might be affected by the need for simple maintenance (rather than replacement) of the filter screens commonly built into home water pressure regulators. You should check the pressure before and after the pressure regulators with the irrigation system turned on (and off) to see if the pressure drops significantly with the system on, and then look for directions on how to clean your filter screens or replace the guts of your pressure regulator, if needed.
You'll also want to re-check if the pressure has stayed where you reset it to. NOT having permanently mounted (but valved off for maintenance) pressure gauges before and after a pressure regulator is a poor place to save money. At minimum, put in boiler drains with hose threads on both sides so you can attach one of the portable gauges as you used outside right there. But really, having both gauges readable at the same time is most convenient, and makes finding problems much easier.
So, for a short list without replumbing anything yet:
- check the pressure to see if it stayed at 90 as set, or changed again
already. Not staying where set is a sign than you need to rebuild or replace the pressure regulator.
- Now turn on the irrigation and check the pressure (of the house, via
the same hose bibb location.) Does it stay where set, or drop
- Since you mention low irrigation pressure, find or make a place to attach your pressure gauge to the water pipes serving your sprinkler heads (or other water-distributing devices, such as drip irrigation) and check the pressure there. In most system types, 30 PSI is a typical value and involves an additional pressure regulator just for the irrigation.
- Turn off the irrigation, and run water in the house (run the dishwasher or laundry washer, or run the tub/shower) - does the pressure remain as set, or not?
If the pressure with water flowing is not where set, or close (5-10 PSI lower), The regulator is not flowing well, which might mean it has clogged filter screens, or it might be a different indication that it needs to be rebuilt. Or, there may be a different clogged filter elsewhere in the system, since you are not measuring right at the regulator.