I thought I understood what it meant when someone was described as behaving irrationally or holding or professing an "irrational fear", but the more I try to look into what a court or legal people might decide amounted to irrational fear or irrational behaviour, or even what might be understood by the terms by other professionals, the less I seem to understand it, (is it me?!).
Put as simply as I can I believe "someone behaving irrationally, or showing an irrational fear has no reason or justification for doing so",......., and signs that this behaviour might be irrational include whether the individual sought to find or uncover the truth of whatever it might be.
To give you an example I believe I once witnessed someone behaving irrationally, (someone who normally didn't exhibit such behaviour). The person concerned made unsubstantiated claims they had items stolen from them by a close relative, the relative's wife, and all their five young children, (ignoring the fact these people were all known to be of impeccable character). Furthermore, instead of confronting the person they believed had stolen from them they tried to tell anyone who might listen that they had been robbed, and doing so without giving any particulars as to what they thought they had lost. They repeated these allegations over a very long period, and so much so, a number of the deluded persons friends really believed a crime had been committed.
Another odd aspect or behaviour, or irrational aspect was not taking their allegations to the police, if they truly believed something had been taken, and the person I'm described really did seem to believe they'd been robbed and were genuinely upset or distressed about it.
When I tried to tell this person they were completely deluded and acting irrationally (or "Off their head" to give you my exact words!), they were completely unable to understand where they were going wrong, and had made some dreadful mistake.
For the purposes of this discussion please assume (and/or accept) the example I've just given you is just as I've described it, (and if that's a struggle for you, please cite another example where you believe you witnessed someone behaving irrationally, or having an irrational fear, and substitute your example, and we'll try to take the discussion from there).