Butter has more liquid in it than shortening, so you'll want to use a bit less. The great thing about icings is that you can usually add a touch more confectioners' sugar if they are too thin, without affecting their quality of the icing. Let us know how it turns out! ;o)

It's for an icing on a bar cookie - I was initially thinking butter might be too runny, but now that I give it a second thought, maybe not

It depends on what you are making. Results vary considerably, depending on the application. Sometimes butter can be used, sometimes oil, and sometimes lard works just as well. What recipe is it for? ;o)