HTTP, IRC and XMPP are all sent on top of TCP, which is a protocol that provides a bidirectional stream between two endpoints (IP address + port). Under the hood, the data you send is split into separate packets, sent across the network, and reassembled on the other end, so that the recipient just sees a stream of incoming data - except when something goes wrong; there is a somewhat accessible description here.
What that means is that while the application protocol (HTTP, XMPP etc) is different, the underlying transport mechanism is exactly the same. It would be possible (perhaps even interesting) to implement HTTP on top of IRC: an HTTP/IRC client enters a channel, sends the HTTP request as messages to the channel, line by line, a server is present in the channel, reads the request and sends the response the same way - but transporting HTTP over IRC is fundamentally different from transporting HTTP over TCP. The former means layering an application protocol over another application protocol (and the IRC connection needs to go over TCP anyway), while the latter is an application protocol over a transport protocol, which is the way things usually are done (except for various kinds of proxies).
Hope that makes some sense...