"Nach bhfuil sí deas" is correct, it means "Isn't she nice?". "Go" is generally used to form adverbs (go deas = nicely) and is not necessary here.
You could also say "Nach bhfuil sí go deas", but that could be interpreted as meaning that she's well on her way to being drunk!
By the way, don't try to imitate the pronunciation from that link, it won't do your Irish much good.