Your Italian cypress trees are likely dying due to infection with "cypress canker", a fungus disease. The trees may have become more susceptible due to drought stress. The borer holes are usually secondary, after the fact infestation of the dying stem. But they can build up in population and go to marginally stressed trees.
With the 405 behind your property, I understand why you need the dense screening. The freeway was not likely directly responsible, but pollution can be an added stress factor for tree susceptibility to disease.
Remove the dead and dying trees, as well as infected dead branches right away to limit spore dispersal with winter rains. If you have a tree service do this they can grind the stumps out too, so you can re-plant the spaces.
The rain you just had will take the pressure off of drought stress temporarily. Get a good deep watering program going in the upcoming summer and fall dry season.
"Provide trees with proper care and prune off diseased branches. Plant species that are well adapted to local conditions and less susceptible to canker."
"Instead of cypress, consider planting arborvitae (Platycladus and Thuja spp.) or (along the coast) incense cedar (Calocedrus decurrens), which resemble cypress but are less susceptible to or not affected by cypress canker. Avoid planting Leyland cypress in California. Do not plant Italian cypress or especially Monterey cypress in inland areas away from the direct local influence of the cool coastal climate."
For the remaining trees, summer watering is the most important factor. Talk to a local tree service with a "PHC" (plant health care) division, like Bartlett Tree Service (a national chain with good PHC techs). Ask them about preventive treatment with systemic fungicide and insecticide, and fertilization. Start this treatment in early spring, which in Irvine is late February.
If you re-plant, consider Arborvitae (Thuja plicata) 'Green Giant', or the larger 'incense cedar'. These are mentioned in the UC article.