@nonumous,
Perhaps I did not make myself clear.
There is nothing wrong with you asking her out for coffee (you're the one who claimed it was a bad idea, not me). But understand that she may have plenty of people interested in her. That's what happens in bars. People who work there get hit on. A lot. Sorry if that's discouraging but that's reality.
So ask her out. Say, "____ (whatever her name is), I really enjoy chatting with you at ___ (name of the bar). Do you want to go out some time for coffee?" And say coffee, which is a little less of a commitment and you are making it abundantly clear that you don't want to just go wherever she works.