As far as I can tell, mIRC doesn't have any option to pop up something when an url is posted on a channel, it is most certainly a script which is doing that.
You can check if it's a script by using the command /remote off, which will disable all your scripts, if you don't get the popup with remote off, then it's a script.
You may also want to show us what kind of popup it is, just to clarify.