You use:
<a href="irc://the.server.name/thechannel" target="_blank">irc://the.server.name/thechannel</a>
For example,
<a href="irc://irc.myserver.com/chatland" target="_blank">irc://irc.myserver.com/chatland</a>
That would bring you to #chatland on irc.myserver.com