The logical opposite of having an irrational fear (or phobia) is not having that fear. But what is the opposite condition, the condition of having an irrational lack of fear for something that should frighten, such as, let's say, being burned? It's not philia since there's no particular affinity for it either, and it's not courage as stated in this answer since courage is something mustered for a particular situation.
Before every human quirk had an official psych diagnosis, the behavior of a person who did not feel fear when it would be prudent (and thus seemed overly bold, irresponsible, or even, suicidal,) was called "foolhardy".
insouciant: Marked by blithe unconcern. (WordWeb)
But this is more general than unconcern about danger.
reckless — having or showing no regard for danger or consequences; heedless; rash
American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved August 20 2015 from http://www.thefreedictionary.com/reckless
The word folly comes to mind.
the lack of good sense or judgment
a foolish act or idea : foolish behavior
Another word that comes to my mind is bravado.
A swaggering show of defiance or courage.
A false show of courage.
The answer that comes closest was disinhibition, which was in Tim Romano's comment.
In psychology, disinhibition is a lack of restraint manifested in disregard for social conventions, impulsivity, and poor risk assessment. Disinhibition affects motor, instinctual, emotional, cognitive, and perceptual aspects with signs and symptoms similar to the diagnostic criteria for mania.
Foolhardy implies general disregard for any danger, while disinhibition suggests lack of inhibition for some particular thing, since "inhibition" connotes, like "phobia," aversion to something specific.