International law exists in a very real way anytime you're working with an int'l client seeking to do something under US law, but not in the way most think it does. The int'l stuff is merely incidental to a practice, and not the essence of it. Your odds of practicing "int'l human rights" or working on Nuremberg types of trials are near 0. When the int'l community inevitably puts ISIS members on trial, they're not going to scour law students who did well on their int'l law exam to comprise the committee. They're going to pick the best prosecutors and litigators from around the globe. If you could apply a criminal statute to fact then you can apply, for instance, the Geneva Convention to fact.
For example, if you'd like to defend women's rights in India then American law is unenforceable. You're going to have to know Indian law so it'd make more sense to go to an Indian law school. Often times the desire to practice something like int'l human rights law stems from a basic misunderstanding of what int'l law is, and American jurisdiction abroad. Even the UN has limited jurisdiction, and you should really be asking how prevalent such jobs could possibly be if there's no "international tax" to pay for "international law enforcement" and "international prosecutors". In terms of what most people asking the question want to practice, most of the people practicing such human rights and "rules of war" type law are going to be working for governments, and advising the military on what it can and cannot do, but these people are few and far between and have decades of experience. For instance, the main reason we never killed Osama bin Laden before 9/11 despite knowing where he was in 2 different instances was because it was against int'l law to go into a foreign country to arrest him. We could only drop bombs from 30k feet up, and even then it could only be "proportional".