Since the Rain8Net is network based, you will need the XEP to have the M1, PC and Rain8 on the network (attached to a hub/switch or router). If you can get a wire to where the sprinker control wires are, which I assume you can since you really have to with the Rain8 net anyway, IMHO there is a better and less expensive route. In fact, this is on my list to get done in the next week or two.
You will need a SPDT relay. I am getting the Elk M1RB (8 relays). You then connect one leg of the valves to the relay(s) N/O contacts and the other leg of the valve to 24V power and to common. The coil of the relay is controlled by the M1 outputs. So you turn on an output for x minutes which energizes the relay coil which switches the valve associated with that relay on. I have 4 valves and was planning to run a cat 5 cable from the sprinkler valves to the relay board in the M1 closet.
how to by BSR for additional background. You would wire the valves and relays according to the schematic.
Also look HERE
for info on how to create the rules, etc.
If you have the 24V wall wart already, all you need is the relay board which is less than the Rain8 and can still be used for other things. You can also hook a rain sensor/etc. to an input on the M1 to decide when to water. If you have more than 8 sprinkler zones, then you would also need an M1 output expander like the M1-XOVR.
Somebody please correct me if I have misspoken as I have not done this yet, just in planning stages. Hope it helps...