This was pretty easy to do and I completed both mini projects in the garage today in about 2 hrs. The pictures above give you the idea - get a Z-wave 120 v. appliance switch (about $40), a 12-volt relay switch (about $5) and a 120 v. to 12 v. converter (about $20). You could skip the converter and use a 120 v. relay also.
For the sprinkler system, all I'm doing is controlling the valve common (negative) wire with the relay/appliance switch. I then set up all kinds of programs on the sprinkler controller and then used scenes in my Z-wave controller (InControlHa) to turn the common wire on and off at set times, thus controlling the sprinklers.
I used a very similar setup for the garage door opener. Except this time, the relay is controlling the bell switch on the door opener. Then I just set up a scene to turn on the Z-wave appliance switch to throw the relay which activates the door opener. I also had to set up another scene to detect when the appliance switch goes on to automatically turn it off. Very nice! Cool to control the garage door from anywhere when you have your iPhone or computer handy.