Don't get an MAIS just to get one. Go only if it's an amazing, prestigious program. Even then it might not be worth the extra debt. Of course, if you get a fellowship, that's a different story entirely.
I talked to some pre-law counselors about this and, unfortunately, an extraordinary number of applicants write about how much they want to go into international law. So, you will need to distinguish yourself. Do your research, demonstrate that you know what you're talking about. If you have a skill that uniquely qualifies you (like you're fluent in a foreign language--particularly one that's really in demand), be sure to play it up. Since you have a year left of school, you might consider developing some such skill.
Regarding rankings, I would read faculty profiles at schools in which you are interested and see if they're a match for the particular niche of international law you want to practice.
Best of luck!