As a general trick, you can ping the broadcast and after a few replies check the arp cache.
ping -b 192.168.1.255
Replace 192.168.1 with your network leaving the .255 (broadcast).
Of course, this assumes you know it’s IP.
Update: The reason you are pinging the broadcast is most devices will reply (instead of targeting one IP). If you know the OpenSprinkler IP you can use that instead.