Actually, it would probably be best supported as:
<a href="irc://irc.server.com/#chan1" target="_blank">irc://irc.server.com/#chan1</a>,#chan2,#chan3 [or]
<a href="irc://irc.server.com/#chan1%2c#chan2%2c#chan3" target="_blank">irc://irc.server.com/#chan1%2c#chan2%2c#chan3</a>
