What comes to mind when you think of the word "irrational"? It has a very accusatory sound, and doesn't instantly appeal to most people the first time they hear it. It only appealed to me after I was able to apply the notion to my own behavior, with positive results. And it seems more precise than colloquial phrases like "human folly". That said, the word "irrational" is still very vague, and the more often it becomes used by people and groups with different ideas of what constitutes irrational behavior, the less clear it becomes. Just as one man's junk is another man's treasure, one person's definition of irrational behavior is not the same as another's. So long as an explanation is given for why a particular thought, desire, or behavior is irrational I think these difficulties can be avoided and the reader can come to her own conclusions. In my opinion, it is easier, and sometimes more immediately fruitful, to spot out instances of irrationality than examples of rationality, but both deserve equal attention.
There is an intersection with these ideas and the skeptical and atheist movement. (Before going further I should re-emphasize that all these terms are equally vulnerable to misappropriation by groups with opposing ideological agendas. Remember how "compassionate" used to be a good word? It still is, but not everyone means the same thing.) It is wise to be skeptical of our motivations when they are likely to be irrational. And the atheist movement is the result of putting religious claims under the critical lens of science, philosophy, and humanitarian concerns. There are voices on the internet calling attention to these subjects. Dan Ariely is a behavioral economist whose focus is irrational behavior, which is also the subject of his last two books. A group of bloggers contribute to "Irrationality Itches", which appears to have been dormant for the last few months. Another blog called "Human-stupidity" appears to come from the political right wing (or libertarianism, I haven't taken a close enough look) in his views of irrational behavior. And I cannot mention irrational behavior without also talking about the psychologist Albert Ellis, who targeted irrational ideas as the focus of his therapeutic work.
So how exactly do people exhibit irrationality? This could be the subject of a long series of posts, if they ever get written. But to spend my time on that right now would be irrational for me to do, in light of other demands on my time and energy. In the meantime I'd like to direct you to the other resources mentioned above, and welcome hearing any of your thoughts on this subject in the comments. I should mention that I don't think irrationality, on the face of it, is bad, but when it goes unnoticed masquerading as rational behavior it can have very serious and harmful consequences.