Before continuing, much of the how's and why's of IRCM are going to be either speculation, or classified. Nations will not freely discuss their actual methods of defeating opposing nation's IR capabilities. Therefore, actual answers will be difficult to give. However, here is an excerpt from Wikipedia regarding a specific missile warning system that may be found on military aircraft. This represents one possible method used for certain types of aircraft.
"The AN/AAR-47 Missile Warning System is a Missile Approach Warning system used on slow moving aircraft such as helicopters and military transport aircraft to notify the pilot of threats and to trigger the aircraft's countermeasures systems.
The AN/AAR-47 Missile Warning System is a Missile Approach Warning system used on slow moving aircraft such as helicopters and military transport aircraft to notify the pilot of threats and to trigger the aircraft's countermeasures systems.
The AN/AAR-47 passively detects missiles by their infrared signature, and uses algorithms to differentiate between incoming missiles and false alarms."
https://en.wikipedia.org/wiki/AN/AAR-47_Missile_Approach_Warning_System
