Use rainwater to partly supply a sprinkler irrigation system?
Simplest 100% legal way would be to make it use rain water directly from storage containers, and then a float valve that fills the rain barrel with city water if the water level gets below a certain level. This would also allow you to maintain an airgap between potable and non-potable water sources.

You're losing the free water pressure from the city water, but you already will need a pump large enough to handle the unpressurized rain water so you're using the pump for both water sources. 

Also hook up a 24 VAC electric operated water solenoid valve that is controlled by your irrigation controller that shuts off the city water to the float valve to make sure that if the float valve gets stuck open that the water won't flow if the irrigation system is off. Maybe getting a 120 VAC one would be better if you make it operated by the same control that turns your pump on and off, instead of with your irrigation controller zones. 

Typically that would be called a master valve in an irrigation system, but this is a little different but mostly the same idea.