I'm guessing there's more to the message, probably something of the lines of "riskware" or "not a virus" mentioned?
Every now and then antivirus companies update their virus definitions and (seemingly at random) decide to ring alarm bells that mIRC is a virus. Usually they mention in the message that mIRC isn't actually a virus but that they consider it riskware or a possible indication of a virus. The reason being that in the past there have been trojans that have used mIRC.exe as part of their payload to send information about the infected PC to an IRC channel. Obviously if you've intentionally installed mIRC then this isn't something to worry about and you can tell Norton to ignore it.
