What we call irrational fear is somewhat plausible because there is always some real danger involve with it. Like if you have a phobia of elevator there are people who die every year in an elevator, is just that it is a very small chance, so it is worth taking that risk.
If you had a past negative experience or a nightmare with an elevator, your body is going to anticipate that danger as a form of precaution. Which is a very important thing, because if you are wrong about it you are going to be dead. And that it is a very rational survival instinct.
RE: Irrational fear doesn't exist!