I seem to remember at least one virus scanner's heuristic updates did at one point identify a string that mIRC contains as belonging to a virus.
As long as you are certain that you have a good, clean copy of mIRC (from
http://www.mirc.co.uk/get.html), I understand you can set the virus scan to not check (or perhaps ignore postives on) a given executable.
However, I also believe that some strings that are checked include backdoor scripts, so you may want to verify that it is the mIRC executable that is triggering your virus scanner, and not some rogue script you inadvertently installed.
HTH.
