Yes, the easiest way is to just get a cheap router (you can also choose to hide the SSID so it’s not discoverable by other devices).
Technically if you can modify the firmware, it’s completely possible to change the firmware so that it works in AP mode too — in other words, all functionality can be supported when OpenSprinkler is in AP mode, hence itself can be a ‘router’ that your phone connects to. However, the firmware currently only supports WiFi configuration when the device in AP mode, and doesn’t expose any other functionality in the AP mode — but enabling these is a fairly simple firmware change.