The easiest way (well, depending on how you look at it) would be to place an irc:// link on your page (irc://server:port/chan), this opens the users irc client (or sends an instruction to one currently running) which will have them connect to that server and join your channel.
Another way (and you can really use both ways, which gives your users a bit of choice) is to host a java applet on your site which will take the place of the IRC client, which saves your users a bit of time/hassle (little hassle really) downloading an IRC client. As I've no experience whatsoever with java clients I can't provide any recommendations.
Hope that helps