the previous answer isn't entirely correct.
the problem here is what you mean with 'rather than in a java plugin in web browsers'.
if a website has installed a java webchat on their website, without giving the link to their chatroom in normal format ( <a href="irc://irc.network.net/channelname" target="_blank">irc://irc.network.net/channelname</a> ) then the answer is: NO, you can't, unless you check the source code of the page and do it manually... (probably you can write a DLL for opera or extention for firefox to do this automatically though)
if the IRC link is given, open up mIRC, go to options > irc > catcher , and enable "enable support" under 'chat links'. with that done, if you enter an irc:// link in your favorite browser it should open mIRC for it (in case of firefox you might have to add the protocol to the options but I'm not sure about that..., in case of opera, you will have to set mIRC as default application instead of opera itself for irc:// links)