As far as I know the Arducopter project does not use IR sensors for anything in the project officially. Only the ArdupilotMega hardware is used used in the Arducopter project.
IR sensor like the thermopiles used in the Ardupilot project is not compatible with Arducopter. IR sensors like the Sharp ones have an accurate maximum distance of 150cm which in a quick descent would give very little time to sense, calculate and adjust the PWM to slow (and hopefully stop) the Arducopter.
You could try to create your own IR system though using FMCW concept with a ATTiny sampling the bounce.
Do you which websites i can get it?