I don't have access to IPv6 currently, however, as far as I recall, none of the features in mIRC that listen for incoming connections support IPv6. Normally, to listen on a specific port for incoming connections on a dual stack system, an application would need to open two sockets, one for IPv4 and one for IPv6.
That said, for users with a NAT router, like me, incoming connections on a specific port would need to be forwarded to a specific local address and port. My dual stack NAT router only supports forwarding an incoming port (with no provision for specifying the incoming address, ie. IPv4 or IPv6) to a local address and port, so it may not even be possible to forward incoming IPv6 connections.
If at some point I get access to an IPv6 connection, I will look into this.