The fun with being an International Relations major is that there really is no plan B as something wholly distinct from what you're up to. I was actually talking with my university's career councilors yesterday, and basically IR Majors can go into any industry so long as a given company in that industry does something internationally (and given all the global trade and foreign direct investment going on today, that's more or less everywhere). So i could work for Firestone tires helping coordinate with the Japanese company that owns them, or work with Ford associating with their plants in Russia, or even stay at my own University, just work in the Study Abroad unit (very unlikely, but possible)
My main plan is to either go teach English in Japan (though that would only be a short-term gig, 1 to 5 years depending on how much I like it and how well I do) or to get into the US Foreign Service, but i'm going to try everywhere I have the time for in my Senior year next year, so anything that isn't the JET Program or the Foreign Service is Plan B, but it would all still be relevant