I think there's overlooking the essential issue:
There is irrational fear and there is a rational fear. We - over generations - have made rational fears irrational. And developed irrational fears too.
Yeah, sounds odd but let me explain.
Rats: associated with refuse and disease, these are actually clean and perform as nature's 'clean-up crew'. Like vultures. But the (past) rational fear of disease - indicated by rats - is now held irrational with our increased knowledge of the creature and likelihood of encounter. Fear of? Rational once but now not.
Spiders: depending on location, spiders may or may not be a real danger. But their appearance is considered unpleasant and parents behaviour informs the child so the child grows up not liking the appearance of a spider. This fear is close to being irrational as in most cases, there is no danger presented by said creature. Fear of? Rational in certain circumstances but now not.
Sharks: Aha! Sharks are dangerous therefore the fear of them is rational. While you are at risk from shark attack. If you are in no risk of shark attack then the fear of them is irrational.
Fear of? See above.
I've heard explanations of those who fear spiders etc. that it's the appearance of them - the tail (?) in rats, the way they move in spiders. Now this is harder to rationalise.
I'm really interested in truly irrational fears. The "why are you scared of that" moment. Someone of my acquaintance has an absolute, gut-wrenchingly abhorrence of seeing ships sink, such as watching a film about the Titanic. Just can't bring herself to see it. She's been on ships, she's a good sea-traveler ... but it's the visualisation of it sinking that makes her almost faint. I posited that she's a reincarnation of someone who died in a sinking but, of course, I have no evidence of this. But it might be an interesting avenue to analysis of "irrational fears".
Bottom line is that I find it fascinating to examine what someone is feared of and why.