I disagree with the idea that irrational fears are simply ignorance. It's not as if someone simply says, "Oh, I am scared of heights. Oh well." People have a tendency to self-explore and evaluate who they are as a person, so most would know of what the fear is rooted in. That said, even when they figure out what it is rooted in, their response is still unreasonable to the situation and difficult to alter. That's where a fear becomes irrational.
I was coming from the idea that the fear itself - be it of heights, mannequins or tomatoes - was irrational, which doesn't seem to hold water, as you seem to agree. If I were to take up your definition of an irrational fear - that it's the inadequate response to those objects that matters... It makes sense. Thanks for peeling some of my own ignorance off me.