How can I prevent the receptacle in my yard from tripping the GFCI used to power my sprinklers?
You can use multiple GFCI devices on a single circuit, you just cannot connect them inline with each other.  If your current wiring looks like this, where the yard receptacle is connected to the load side of the GFCI receptacle in the garage.



You can change the way the second receptacle is supplied, by connecting it in parallel with the existing GFCI receptacle. Then you can swap the receptacle in the yard, with a GFCI receptacle.  



In this configuration, the GFCI in the garage will only trip if a ground-fault is detected in any device plugged in to it. While the GFCI in the yard will trip if any device plugged in to it, has a ground-fault.

If the GFCI protecting the circuit is a combination GFCI/circuit breaker, you'll have to swap it out for a regular circuit breaker.  Then you'll have to protect individual outlets appropriately, using other GFCI devices.