DIY Network

would installing an underground sprinkler system be harmful to existing large trees that are in the yard?

No, but with those trees, the sprinkler system maybe in danger from their roots. You will find out when trying to install the system, how the roots system of the trees will affect the sprinkler system or not.

Bowiegreen, you can see how to run your lines under tree roots. You will need to be very careful not to knick or damage the tree roots.
http://imageevent.com/mdvaden/treerepair?p=23&n=1&m=-1&c=3&l=0&w=4&s=0&z=2

Try and keep the water away from the trees as overwatering them is not good for them. You might want to look at these sites about tree roots.
http://edis.ifas.ufl.edu/WO017
http://www.ext.colostate.edu/pubs/Garden/02926.html
http://www.treesaregood.com/treecare/trees_turf.aspx
http://www.treesaregood.com/treecare/mulching.aspx

How to water mature trees.
http://cals.arizona.edu/pubs/water/az1298/

Newt