I've lived in Girrawheen for a few years so I can speak from personal experience. As everyone knows, the suburb doesn't have a great reputation, although that is slowly changing. The crime rate has plummeted dramatically over the last 10 years - Girrawheen today is about as safe as the surrounding suburbs. I see people out jogging or walking the dog in the evenings regularly - a sure sign that the residents feel safe.
The crime that does happen seems to be mostly petty crime like bored teenagers looking for iPods in parked cars. So secure your hours with an alarm (or a dog), take the usual precautions such as locking your doors, and perhaps avoid visiting the ATM or petrol station at 3AM, and you shouldn't have much to worry about. If you're moving here, try and pick a street where the houses look well maintained. The area is being rapidly redeveloped so it's only going to improve.
I honestly would prefer not to go wandering about on my own in most local Girrawheen streets, and I only drive by occasionally. But, like anywhere once you become familiar with your own streets it is probably not too bad.... but not after dark, for sure! The crime rate is nothing to boast about, so exercise caution and get good security on your own home.