Sprinkler systems are covered under the Illinois Plumbing Code issued by the Illinois Department of Public Health. Here is a copy of the section of the code that refers to backflow.
DEPARTMENT OF PUBLIC HEALTH
NOTICE OF ADOPTED AMENDMENTS
Section 890.1140 Special Applications and Installations
a) An atmospheric vacuum breaker shall be installed between the control valve and the fixture and in such a manner that it will not be subject to water pressure, except the pressure incidental to water flowing to the fixture. An atmospheric vacuum breaker shall be installed on the outlet side of the control valve.
b) Flushometer Valve. Flush valves shall be equipped with vacuum breakers installed on the discharge side of the flushing valve with the critical level at least 4 four (4) inches above the overflow rim of the bowl or 4 four (4) inches above the top of the urinal. (See Appendix I: Illustration D.)
c) Flushing Tanks. Flushing tanks shall be equipped with anti-siphon ball cocks. The ball cock shall be installed with the critical level of the vacuum breaker at least one one (1) inch above the full opening of the overflow pipe. In cases where the ball cock has no hush tube, the bottom of the water supply inlet shall be
installed one one (1) inch above the top of the overflow pipe. (See Section 890.650(d).)
d) Lawn Sprinklers. Any lawn sprinkler system connected to a potable water supply shall be equipped with a reduced pressure principle backflow preventer assembly (RPZ). The RPZ may be located outside provided it is protected from freezing or is removed at the end of the season, and it conforms with Section 890.1130(g)(1).
This link will give you more information from the Illinois EPA: