It is possible but requires alot of knowledge of HTTP/1.0 and HTTP/1.1 protocol as well as indepth knowledge of mIRC sockets.
Very simply, the process is as follows:
1. Socket connects to your forum host (www.whatever.com
2. Socket requests your login page
3. Socket sends data to the login processing page via POST
4. Socket *usually* (not always) recieves data as a COOKIE
5. mIRC needs to store the COOKIE
6. Socket requests your forum page(s) and at each time posting back the COOKIE.
7. To post, mIRC repeats step 3. but only to the 'post processing page'
If you have sound knowledge of mIRC sockets, the rest (protocols) can be obtained via a nifty program called 'Ethereal' which basically monitors/records all data sent between you and the server, so you can get the protocol from there.