If you need a secure encryption there is a great mIRC add-on out there called Mircryption. It utilizes Blowfish-CBC which up to today has not been infiltrated.
You have to perform the following steps to set it up correctly:
Get the newest Mircryption Lite version from the developers site and start the installer. Be sure to install the add-on in your working directory.
Check the path by typing “//echo -s $mircdir” in any of your mIRC windows.
After the installation is finished, you can load the script by typing “/load -rs1 mircryption/mircryption.mrc”. This will install the script with the highest priority to make sure it does not interfere with other scripts.
There will be a popup asking for permission to execute the on-load code of the script. You should allow this by selecting “Yes”.
The next steps have to be performed while being connected to a server and inside a channel . Open the context-menu of the channel (right-click on empty space). There will be a new menu entry called “Mircryption”. Click the sub-menu called “Set encryption key for current channel”.
On your first usage you will be asked for a secure password. You have to type in this password once per mIRC start. Use something memorable but long enough to be safe.
Then you have to enter the keyphrase which will be used to encrypt and decrypt messages to and from other users. Please be careful not to exceed the 50 character limit, as you will get unpredictable results and can not work together with other implementations of Mircryption. Also make sure to prefix your key with “cbc:”
You will see a status message documenting the success of the last step.
You will notice that your nick will look different when you use encryption. With the standard theme you will see square brackets  around it while normally you would see angle brackets <> there.
When you start mIRC again, you will be prompted for the master passphrase you entered before. This will happen only once upon joining one of the encrypted channels.
You can always disable the encryption by using the “Disable current channel encryption temporarily”. You can activate it again by using “Re-enable current channel encryption”.