it's a replace of characters, but if you don't know the right decryption key (a number) you can't decode it

So obfuscation then? Obfuscating is relatively easy to crack though. Depending on character values, and the key, an obfuscated string could be cracked in a fraction of seconds.

Sunny does have a point.

t116uy
y88op0
foobar

Could all be the same thing, just a simple replace, but after a while of seeing encrypted chatter people could gather enough of a database of it, and reference common cryptography information to crack it pretty quickly.

One solution would be to constantly switch the key, but it would have to be communicated to the person(s) you're speaking to, which would require a private/semi-private conversation with them anyway where the public could not discover the key.

Not sure if mIRC allows manipulation of raw bytes, but stepping down in the data infrastructure would be a much more secure way of encryption. Encoding bytes based on a reversible byte cipher with a key is a similar technique used by other well known encryption algorithms such as Blowfish.