"She is not excited- she is sad."
Translation:Níl sceitimíní uirthi- tá brón uirthi.
I wasn't familiar with ar bís, but it looks like it can be used to describe anticipatory excitement (it is translated as "on tenterhooks" in some sources), whereas you can be excited before or after an event , and if you were excited because you have just met someone famous, or because you just won a prize, then ar bís wouldn't be appropriate.
This particular sentence, though, could be talking about excitement in response to an event ("That was brilliant!"), or in anticipation of an event ("I can't wait!"), so ar bís could be used, though it would narrow the way the statement could be interpreted.