You shouldn't have to open the actual 'irc server' ports (6667-6670, 7000 etc.) as you 'connect out' on these. If your system is comprimised and you have irc server ports open you could be used as a drone server. The ports you usually have to foward are:
113 for identd: the server 'connects back' to this port as you connect to it and requests your 'user name'. If you do not have this forwarded you will have a ~ before your user@ section of whois (or usermode +n on austnet). Added penalties can include stricter cloning limits, probably being banned from a lot more channels and your initial connection to the network can be a *lot* slower (as the identd request from the server has to time out).
59 (or whatever port you specify) for dcc server: If your friends have not correctly set up their port forwarding they will be unable to dcc send or chat with you because you will not be able to 'connect back' to the listening port they specify. By enabling your dcc server you can essentially have your friends connect 'to you' to initiate chat and send sessions, you can also receive chats and files whilst not connected to the irc server. Be warned that many trojans use this to initiate chats and sends claiming to be from NickServ, X and other common services to ask for passwords or send you hostile files. It may be better to change this port (/help /dccserver).
Your DCC portrange: If you do not set this up you will be unable to dcc chat and send to anything but dcc servers (with /dcc send <ip>:<port> etc.). You can find the port range in Options (alt+o) > DCC > Options.
Hope that helps
