mIRC does not store the prefixes, but gets them from the server when you connect.
As a simple explanation as to how mIRC gets this information, irc.x-tab.org (running UnrealIRCd for those who wanna know) sends the information as this:
(from RAW 005) PREFIX=(qaohv)~&@%+
Meaning that:
q (channel founder) would be displayed as ~Stealth`Warrior
a (protected) would be displayed as &Stealth`Warrior
o (op) would be displayed as @Stealth`Warrior
h (half-op) would be displayed as %Stealth`Warrior
-and-
v (voice) would be displayed as +Stealth`Warrior
