Not exactly what you were asking for, but it might do.
From the mIRC help file
Lock
The Lock dialog allows you to lock various features in mIRC.
Ask for Password
If this is turned on and you have set a password by using the Lock button, mIRC will ask you for a password each time it is run.
The Lock Button
This allows you to set a password which locks the options in the lock dialog.
Note: If you hold down the Control key when you minimize mIRC, it will ask you for the password when you try to open the mIRC window later. If you use Control+Minimize with no password set, mIRC will ask you for a temporary password.