I see where you're coming from. Fear is something that just is, its neither good nor bad in itself .. only what we do with it.
Depends on what "irrational" is. Probably has to be determined on a case by case basis. Also, with 3 people there will be 4 different opinions on what is "irrational".
Definitely, in some experiences of mine, fear is overcome by thorough study and understanding. Sometimes its not - I've gone skydiving and still don't like staring over a big drop unless its on a chairlift or mountain with a pair of skis. Just one example, but I'm not sure how much more I can understand heights. I guess I have to disagree that fear only comes from not knowing.