IRC and mIRC are both dangerous in my view.
mIRC is now populated by script writing kiddies who can plant anything on a computer system when you enter their chat rooms. I haven't used mIRC for many years now because the threats were increasing. mIRC is the shareware version of IRC so I assume, (but I confess I don't really know), that the same threats exist in IRC.
Sun's Java often has reported vulnerabilities and these are patched soon after they are notified. But if Java mIRC or IRC allows the transport of scripts, then I suspect that similar security problems will exist.
If you intend to use any of these to chat then it would help to use chat rooms that are fully and constantly Moderated.
Does chatting on IRC thru mIRC pose any danger to computer security?
Also, is there any danger to computer security by chatting through a Java, browser-based IRC chat?