There are no "mirc servers". mIRC is just a program which lets you look at IRC in an acceptable way. If not it would look awfully ugly. A bit like how Internet Explorer lets you see web pages rather than decypher raw website code. IRC is the protocol, so there are IRC servers.
The majority of IRC servers will scan certain ports when you connect, yes. This is due to abuse, and the scanners are there to detect open proxies. If an open proxy is being used then users are automatically banned. Hurrah. If not you're fine. The best thing to do is to allow mIRC in firewalls, port scanning from servers is not a big issue, and allowing mIRC should still allow you to be notified if a user on IRC port scans you.