You cant send to yourself like this becuase your routers not good enough to do the wan to lan redirection, dont worry about it, 90% of routers dont do it.
This is what happens when u send a file
You: /dcc send Bob filename.txt
Your mirc /msg Bob a secret message saying your filename,ip,port,filesize
With that Bob connects to your IP on the Port said and downloads a file naming it filename for filesize bytes
Now if you do that to your self with 192.168.2.2 (or what ever your lan ip was) your mirc can see that IP but no one out on the internet can, becuse thats not an internet IP its your LAN IP.
So you set the IP to being server lookedup, so it becomes your WAN IP (internet ip), then when the person conenects to you, they reach the router (which is the first thing on that IP) and the router goes "wow look someones coming in this port, and i have that port forwarded to 192.168.2.2 I better send this on to 192.168.2.2" and thus you get it. What your pc does with it from there is its bussiness, if your firewalls still up it liekly ignores it.
Only maybe 10% of routers are clever and when you connect to yourself for a file, the outbound connection to get the file is going from your WAN IP back to itself (same IP) so the router goes, "well duh, why send this out at all, ill turn it around and make out its an inbound packet", routers i know can do it, some linksys (setable), some netgears, some dlinks. others i dont know about.
