Houlihan's is right next to the museum - I'm not sure if you leave from the Zoo America exit, can you then reenter the zoo and get back into the park, I'd assume yes, right?
Anyways, so when you leave the zoo, you'll see Houlihan's from the zoo, it's over the bridge and on the right handside, very easy to find! (walk up Park Avenue towards 422)
I don't think I would leave the park mid day to go eat at Red Robin, that would be a lot of hassle, leaving the park, going to eat, coming back, re-parking, etc...