If you are using Windows Vista, 7 or better, you can just "pin" the mIRC icon to your task bar when it is running. That way, it will start when you click on that icon, and then it will only activate it any time you click on it again from then on.
Most programs don't have a single-instance option. It's pretty easy to not open a second instance of a program if you simply do not want to open a second instance.
But I only mention this in case you didn't know how to pin icons to your taskbar. Khaled may decide to add a feature for since-instance behavior.
