motion activated led array
Your PIR sensor (Passive InfraRed, that's what your HS3600 is) contains a photodetector which controls a relay (or, more likely, a triac). The photodetector is likely to have 3 connections: GND, Vcc and signal out. Signal out is a logical signal which (via a few extra parts for the delay) controls the relay. The relay is probably directly connected to the mains. You have two options:  

1) either disconnect the relay from the mains and use it to switch the batteries of your IR light, (doesn't work if it's a triac instead of a relay),
2) or use a transistor to switch the batteries, and drive the transistor from the relay's control.  

warning
the whole circuit will be live, that is directly connected to the mains!