If you really want a native Win32 / IPv6 / IRC Client, then let me suggest you try TinyIRC. It's relatively new, it's beta and only has the most basic IRC functions, but it's nice to look at and natively supports IPv6.
TinyIRC Homepage..
Another Win32 IPv6 IRC Client is KVirc which is ported from linux. Most linux/unix IRC clients like BitchX and irssi already have IPv6 support, which is about as much as i can tell you about linux/unix as it is not my field of expertise.
KVirc Homepage.
If you insist on using mIRC to connect to an IPv6 server, the only way to do this is through an IPv4 to IPv6 bouncer that runs locally on your computer (not neccesarily, but it's the most common setup). The only one i've heard of so far is AsyBoV6 (there probably are others). It's pretty straight-forwarded to set up, but there's a guide
here.
AsyBoV6 Homepage.
A
list of IPv6 IRC Servers. I hope this helps.
PS: I'd also like mIRC to have IPv6 support somewhere in the future. Most people don't get why, but i guess i just like to play around with new stuff