You need to use a file archiving program to extract a RAR file. WinRAR and WinZip are two popular programs that both support RAR files.
As RusselB mentioned, mIRC isn't designed to be a filesharing program - nor is the IRC protocol in general. What mIRC is good at is chatting. But if you just want to share files you'll be much better off using a dedicated filesharing program/protocol.
Spelling mistakes, grammatical errors, and stupid comments are intentional.