When you login to a website you're given a cookie (or cookies), you need to store this cookie and pass it on every request afterward. If using Chrome you can press ctrl+shift+i to bring up the network panel, this will allow you to view the header and post data when logging in. Recreate the request in mIRC with the appropriate fields.
You pass the cookies in the request as:
Cookie: cookie1=value1; cookie2=value2;