Often people are not as irrational as you'd think. It's just they only connect the dots one or two at a time. So, much of the knowledge they already have is not questioned. More often than not people appear to be irrational because they have learned something that is either false or perhaps true but very different to a false belief you hold. They appear irrational when assessing the same situation as you but arriving at different conclusions, because they are holding to a very different set of assumptions. People (my self included) are not very good at being aware of our underlying assumptions.

I guess that is being irrational given that you are not taking all the assumptions into account, but it's the level of rationality that most people operate at.

However if your talking about an emotionally charged situation, yeah rationality be damned emotions are running that show.