Installing an irrigation system IS the number one thing you can do to improve the overall health and beauty of your landscape. Just look at Arizona. They took the desert and made some of the most lush golf courses in the country. Irrigation is good for the trees, too, not just the grass. But, be careful, there are some issues.
Number one. You are going to damage the tree when installing the irrigation system (for existing trees that is). Installing irrigation means digging trenches. Digging trenches means cutting roots. I would like to offer a couple pieces of advice on this topic. First of all, keep as far away from tree trunks as possible when cutting the trenches. Second, if you are going to get anywhere near a tree trunk, use an airspade first to make sure you aren’t cutting really big tree roots. And, third, fertilize the trees to help them rebound from the root damage.
The next thing you really need to watch out for if you are installing an irrigation system is the drainage. If the water has no where to go and it is constantly building up you will drown the trees. You can kill a tree by keeping the soil constantly saturated. If the ground takes more than 3 days to dry after a decent rain then you might have a problem.
The last thing is to make sure you get it set right. I wish I had some good words of advice here that everybody could use to set their system to. The reality is that all sites are different and require different settings. Ideally, longer and infrequent watering is better and should be the starting point to tweak the system. However, rocky hillsides that drain fast will need more frequent watering intervals and in some cases you will be able to turn off your system entirely during the cold season (all three weeks of it!). You really just have to have a green thumb to get this one right.
Do everything right and your irrigation system will be a huge success for you landscape. Even the native plants that are well adapted to the hot, dry summers will appreciate it.