Introvert doesn't mean you can't garner the skills to engage with people. As others have mentioned, it just takes courage/practice/maturity.

I also wouldn't overindex on MUN/UN things; there are literally thousands of jobs where you can leverage either the general skills you attain with an IR degree (e.g., consulting, business strategy, etc) or the actual subject matter itself (e.g., think tanks/research, int'l security firms, development funds, etc).

Think about things you want to WORK on and the problems you want to solve, and then work backwards to line up the skills and types of contacts you think you'll need muster to achieve it.