Little late on this one, hope your pipes are fine. Most in-ground irrigation systems won't be phased by a little overnight freeze, usually you would have to see at least a few days of solid freezing temps to do any damage. That being said, pipe that is above ground is definitely more at risk than buried pipe, and pvc is more at risk than poly. Brass fittings like ball valves and duel checks can also be more at risk as they tend to be exposed more. Reducing or eliminating the water in pipes is the most effective way of combating potential frost damage. Shut off the water source at the inside, and open all valves (by the bleed screw) and hose bibs to allow as much water/ pressure to drain out of the system as possible. If you are still concerned, insulating the pipes from cold works too.