You may have a quick opening valve for your irrigation system causing a quick change in the water flow velocity somewhere. (And/or something in the system that's stopping the flow suddenly.) This causes a hydraulic shock wave to travel through the water, which can result in a banging noise when there is no where for it to go.
A water hammer arrestor which incorporates an air cushion, is commonly used to help soften the shock.
The arrestor should be placed somewhere between the valve that opens the irrigation system and your PRV.
Out of curiosity, have you ever noticed that the water hammer possibly did not happen while there was no water being used in the house? Just thinking that there may be a remote chance that the PRV was part of the problem while there was water flowing through it.
There are different size water hammer arrestors but I believe it may be difficult to determine the adequate size required. You can try one that is not too expensive and hope for the best.
You could use a section of vertical pipe with trapped air in it, as many of the old timers used back in the day. However, since there is no physical separation between the air and the water, the air eventually gets absorbed into the water and you lose your air cushion. So if you wanted to consider making up say a larger air cushion with a section of pipe you should consider valving it so as to allow you to isolate and drain it occasionally to restore the air cushion.
