irssi seems to default to IPv4 addresses before IPv6. That's wrong: IPv6 should always be tried first, to help with the conversion to IPv6.

In fact, irssi should just respect the order provided by getaddrinfo(3). It is already configured to use IPv6 first if both sides have native IPv6, but use IPv4 first if both sides are tunnelling. See RFC 6724 (the new version of RFC 3484), which is implemented in most libc.