Well i have nil experience of geology. But for IR i would tell you the following few things

1) Its not a walk in the park. It requires a good amount of time to get familiarized with the concepts.
2) IR and cramming not a good combo. This subject requires answers in a very objective manner.
3) Though course is lengthy but once you start. It will help you in Current affairs, Pakistan Affairs, Essay, USA History and International law as well

For Beginners "IR by Joshua goldstein" will help understand the basics about theories and IR in general

My suggestion, go through the syllabus and if you feel confident enough then go for it other than that may be geology (mazak kay ilawa mujy geology ka nai pata kuch be)
Baqi best of lcuk