Irrigation systems are best installed before a yard or garden have been planted. They can be installed after the fact, but your yard will have to be torn up and restarted after the installation. 

For larger lawns, you will want to make sure you invest in pop-up sprinkler heads that can spray a good distance. You also want to keep in mind that pop-up sprinkler heads are prone to malfunction and lawn mower damage. It may be an investment, but you want to ensure you buy the best quality you can afford. You will also want to have the system serviced every spring and fall and, in cold climates, you MUST drain the system before winter. 

Here are a couple things to keep in mind if you have the system professionally installed: 

1) Make sure the contractor installs a rain sensor - this will prevent your system from running during the rain.

2) You may want to have them install a new high-tech timer.

3) Talk to them about installing a backflow prevention device approved by the local plumbing inspector - this keeps any water that enters the sprinkler system from reentering your drinking water supply. 

4) Insist on getting a certificate of insurance that includes liability and workers' compensation coverage. 

5) Make sure everything is written out in a contract - materials, warranty, and start and completion dates.