Do you want the astable timer to keep running indefinitely? Or only for a period of time?
Triggering it shouldn't be a problem. You'll probably want a second 555 wired in monostable mode, with the trigger (pin 2) connected to the collector of an NPN transistor. The emitter should be connected to ground, and the base connected to your PIR sensor output. When the PIR sends a positive output, it switches the transistor which triggers the monostable 555. The output of that can then be used to power your second 555.
There are quite a few different options, it would help if we knew exactly what you were trying to do.
Regards,
Matt