IRC doesn't have a user interface, it's a protocol. The interface varies based on which client you are using. One can make an IRC client that looks like slack.
No, it can't. No multiline messages, no threading (a lot of people want it, though personally I think it's a terrible idea), no embedding images.
IRC is Not Good.
Image embedding depends on the client you use really. For example The Lounge [1] can display information about links and show image previews like Slack does.
That's very different. In Slack, I, as the poster, can decide whether to show a preview (it might be pointless and take up space), and add metadata to it. It's also searchable by _anyone_ in the chat history. The uniformity of these features matters, and you need protocol support for that.