Look, IRC is a protocol. It can be used for many things.
To engage in filesharing through IRC you use a special client like:
http://krimpy.sourceforge.net/
The IRC protocol is meant to use for text chat. It's a dangerous network and is often used for hacking more than anything else besides chat.
Most hosts forbid any IRC clients from their machines.
Some IRC clients have malicious backdoors that they will use to take over your computer. The most notorious one simply claims they are unintended "bugs".
Now on to your question:
MIRC is not a file sharing program. It can be used as a file sharing program but so can most IM clients like ICQ etc.
Since I do not use IRC for file sharing and do not use MIRC I can't tell you how it works.
And the authors won't either, since as they say:
"People, mIRC isn't a file sharing program...
It's a text chat program.
We dont help with illegal filesharing/warez on this forum because:
-downloading files from strangers is a great way to get trojans/virii
-it is against the AUP (acceptable use policy) of some networks/providers
-involves mostly files that are illegal to share."
There are many better ways to share files. My advice is to give up on IRC file sharing. It's dangerous if you do not know what you are doing (and you don't, or you'd not ask this).
Now if you are very interested in trying here's a tutorial:
http://www.dslreports.com/faq/4844
Note that I don't think you will like it. It's not like Kazaa or anything.