If you're capable with phpBB you can install the blank page template mod and then include the pjIRC applet to connect to an IRC chatroom
That's what I used to use...
Recently I've redeveloped the site and made seperate pages for the non-phpBB pages and included the phpBB session accross it, then used $userdata[username] variable in the applet code to allow for the people who are already signed in to sign in as their username
That's still using pjirc...
Have a look in the Knowledgebase ( http://www.phpbb.com/kb/
) and look under improvements - > Something about sessions to learn how to include your sessions across it
is the link to my site's chat room for example (using the if signed in statement so non-signed in people can choose their name before going into the chat)