If it comes with an IR remote, you can first learn what signals from the IR remote will trigger what function, then use your pi to send these IR signals out with an IR LED to control them. Most classroom automation systems that involve projectors, VCR/DVD players etc use this technique.
To sense the remote and send signal out, I recommend an Arduino UNO. The pi is likely not able to generate reliable 38KHz to 40KHz IR signals with perfect timing.
I can point you to some libraries and code for Arduino if you are interested. I once made an Arduino LED light show kit:
http://www.inmojo.com/store/liudr-ardui ... -show-kit/
This is only the receiving IR remote side of the solution. Once you know what signal means what to the AC unit, you can send them out with an IR LED.
Arduino data loggers, user interface, printed circuit board designer since 2009, RPI 3B 2B 2B Zero Jessie, assembly/C/C++/java/python programmer since the 80's