I would like to connect to to irc, icq, sip, etc services using WebSockets. Assuming I have some sort implementation of those protocols in JavaScript ? Is that possible? I don't seems to understand limitations of WebSockets comparing to regular sockets.
No, you can't, at least not directly.
WebSockets allow real-time messaging between a browser and a WebSocket server, but they have their own layer 7 protocol for encapsulating those messages.
They don't provide access to a pure TCP (or UDP) socket over which you can implement existing protocols.