ok, I admit I'm a bit of a purist, but...
It's IRC, not mIRC, mIRC is just another IRC client, you do not log into mIRC with your Undernet Username, you in fact login to X on Undernet's IRC Network (I for one do not and can't use mIRC as it does not have a mac version ;)
As for the usename suspension, the best place to ask about it is email@example.com
. It is however most likely you were caught by our process for finding attempts to register multiple usernames, which is not allowed.