This was resolved by making sure that mIRC only resolves IPv6 connections when necessary, eg. if mIRC is binding to an IPv6 address, or if IPv6 support has been enabled in the ports dialog, or if you are using the -6 switches, and so on. As far as I am aware, this covers all of the situations where IPv6 can be used. If I have missed one, or if it is not working for you, please let me know.