The check_ircd script uses low-level socket library calls and as such
cannot check IPv6 IRC servers.

It would probably make sense to rewrite this to use an IRC library.

Net::IRC is deprecated these daays (#608583) and claims that
Bot::BasicBot or POE::Component::IRC is the best way forward, but both
of those may be a little heavyweight, I suspect.

<http://sourceforge.net/tracker/?func=detail&atid=397599&aid=2114022&group_id=29880>

may contain a working patch to add IPv6 support to the existing code,
although needing to pass the -6 flag isn't ideal.

Cheers,
Dominic.