If the old system used "standard" sprinkler wiring then the wires will be color coded. You can then match the wire color at the solenoid. Common wires are usually white and power wires black. There should be a transformer plugged in somewhere if there is still a 24v power wire. find it, unplug it, and remove those wires. The Iro comes with a new power supply that plugs into the unit.
If your wires are not color coded, you can just plug in what you have and do the setup. During that process the Iro runs each zone, so when the sprinkler comes on you now what gets watered on the zone. May not put the zones in the "logical" order, but at least will identify what wire goes to which sprinkler.