A circuit has been tripping for the last couple of days. It is the circuit in my garage (that also feeds the water sprinkler). For years I had no problem, AND THERE ARE NO NEW ADDITIONAL ITEMS PLUGGED IN THAT CIRCUIT. We have noticed "trip" when the sprinkler goes to "zone 2" of the back yard (after it has finished zone 1).

Question: Any advice on how to solve this matter?

Assuming this is a ground fault breaker, and assuming the sprinkler systems uses solenoid valves, for each zone, there may be a nick in the wire feeding zone 2 or or the coil of the solenoid valve is shorted. You may try swapping valve coils first to see if zone 2 will work with another valve coil. If the tripping persists, then the wire is suspect.