Well, some fears are in accord with reality. I fear tigers because they might kill me. I fear big waves because they might drown me. Even though the emotion of fear is not a rational thing, most of our fears are in accord with actual danger.
An irrational fear would be one that is either at odds with what we can know logically, or is out of scale with what we can know logically. Saying "irrational fear" is a shorthand expression.
An irrational fear would be one that is either at odds with what we can know logically, or is out of scale with what we can know logically. Saying "irrational fear" is a shorthand expression.