First you may want to look into any possible reasons the pump cannot meet the required needs. Could be beyond it's capacity, or something else.
If you end up trying to shut the sprinklers down, take a look at what pressure your system is holding when the sprinklers are on, in order to determine shut off pressure.
Would it not be easier to just cutoff power to the existing sprinkler valve(s) rather then adding a new valve. Possibly could be done with just a pressure switch alone, or at most a pressure switch and relay.