first off, mirc is a program. mirc is not irc in the same sense that internet explorer is not the web.
second off, you're way too paranoid.
via irc someone can get your ip, and from there they can guess what operating system you're running (i'm sure they can tell that by other means anyways), and they can check if you have any ports listening. in general, barely anyone is smart enough (or perhaps just motivated enough) to even get that far. and the rest would get that far and then fall asleep. the only real threat would perhaps be someone dcc'ing you a trojan and telling you its some w4r3z pr0ggi3 and then you run it and screw yourself. but if you had some common sense you would know that running executeables from strange people on irc is hardly an intelligent thing to do.
so we can conclude the following:
there are not that many smart people on irc.
the smart ones dont give a crap about hacking you.
even if someone was motivated to 'hack' you, they probably couldnt.
so chill out, there are hundreds of thousands of people on irc all the time, no one cares about hacking you.
oh, and there is really no need for a bnc unless you have: 1. something to hide, 2. something questionably illegal or at least mischevious to do.
so relax and irc like its 1999.