sounds like you need to do some port forwarding in your router, then put those ports in mIRC's options file/options/dcc/options where you see the default of 1024-5000. (I assume your dsl settings arent the problem) Also be sure mIRC is getting the external ip for machine, not internal network one.
when you can recieve but not initiate sends/chats, usually means either mIRC isnt getting the correct ip or the ports are blocked someplace, like router/firewall/dsl settings.