Just had an 8 zone irrigation system installed last fall & was winterized. I have what appears to be a master valve underground in my front yard directly off the main water shut off. It has a yellow handle that is currently perpendicular to the pipe (like a 'T'). The side of the house has above ground cover with a regular hose on/off handle (circular) and two blue handles that are both horizontal (in line) with the pipe.

Above ground housing: http://www.flickr.com/photos/76128187@N04/6836508658/

Below ground valve: http://www.flickr.com/photos/76128187@N04/6836507782/