I have an illogical fear of insects, especially if they are cockroaches.

[sania-baharat]

Is this sentence OK?
I have an illogical fear of insects, especially if they are cockroaches.
I have an illogical fear of being stuck in enclosed spaces, especially if they are elevators.

 

[Raymott]

"I have an illogical fear of insects, especially cockroaches."
"I have an illogical fear of being stuck in enclosed spaces, especially elevators"
You can, and should, leave out "if they are" when you can write them as above.
Sometimes it's necessary.
"I have an illogical fear of people in authority, especially if they are angry." This one makes sense, and you can't leave it out.
"I have an illogical fear or people in authority, especially [STRIKE]if they are[/STRIKE] police."

Cockroaches are insects, elevators are enclosed spaces, police are people in authority. But 'angry' are not people in authority.
Sometimes either form might work.

 

[kilroy65]

I'd say "irrational fear", especially when it comes to phobias.