The libpcap filter syntax is documented at: http://www.tcpdump.org/manpages/pcap-filter.7.html
Unfortunately, it does not directly recognize the IRC protocol. However, Wireshark determines whether an IRC message is a request or response by looking at the tcp port (is the message to or from port 6667) and we can do that with libpcap.
So you probably want "tcp dst port 6667" for your filter.