A PIR detector is quite different from an IR emitter & IR receiver setup.
An IR emitter and IR receiver could be used to make a 'magic beam' which would detect something blocking the beam.
A Passive Infrared (PIR) Detector detects small differences in temperature.
PIR detectors are used to detect slightly temperature differences, for example something warm compared to its surroundings, which detects a persons body temperature.
There are lots of circuits around on the web.
HTH
GB