Ok, I'll be that guy. No, at least not entirely. There are things most everyone will agree are irrational, for example going off looking for the edge of the earth, or looking for an analytic solution to the general quintic. Since there are things (almost) all of us consider irrational, rationality must be at least intersubjective. If I believe the earth to be flat, and so that there is an edge of the earth to be found, I may think I am rational in going looking for it, but I am wrong; I should know better.
But to you, it's rational. Imagine that you believed it so deeply you were willing to kill for that belief. Or, travel around the world to find the edge. Just because you should know better doesn't mean you do. Thinking about this is definitely a mental exersize.
A valid point. Is death then the determiner of what is rational and what isn't?