There are two types of the mini relays: the earlier version used a black 110V AC / 2A relay, and the current version uses a yellow 250V AC / 3A relay. Both would work perfectly fine for driving a sprinkler solenoid, which only needs 24V AC / a few hundred milli-amps. I don’t see any big issue as the voltage / current both fall well below the relay ratings. The back EMF could be an issue, but the relay pins are relatively far away from the rest of the circuit, so it’s probably not a big concern in practice.
Dan’s interval program now provides a plug-in that shows how to control the relay. The OSPi user manual now also includes information about the pin that the relay uses:
Basically outputting a high on that pin activates the relay, and low deactivates the relay.