If you want an IRC client that is as full featured as MIRC you have it with KVirc. I have to admit I'm very impressed by the feel of this client. It really outdoes MIRC in the look and feel department. Not to mention that it has built in encryption for private messages/queries. There is SSL support and support for proxies(like TOR), sound, voice-chat, and has a scriptable interface.
The current 'stable' release isn't portable though, you need one of the development releases. The most current one will be fine.