Go to your local municipality to see if the layout of the irrigation system is on file. Some municipalities may require that a plan be on file.
Carefully poke a metal wire, such as from a coat hanger, into the ground where you think the pipe is located. The pipe should be 6 to 12 inches deep. When probing the ground, begin by going in a straight line from the point where the pipe goes into the ground near your house to the first sprinkler head in the ground. The pipe should be relatively straight, barring any objects, such as trees, that may block it.
Scan the ground with a metal detector, gliding it back and forth over the ground until you locate the pipe. Even if the pipes are plastic, you may still be able to locate metal valves in the line.
Use acoustic pipe locators to find the pipe. This involves placing a transmitter onto the portion of the pipe that extends out of the ground and listening for the vibrations in the pipe. Acoustic locators work best in dry, sandy soil.
Leak detectors are another option. They operate in a similar fashion to acoustic locators, but are more sensitive and designed to find leaks in the pipe.