"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.

Every course and grammar book I have ever read lists about 8 adjectives indicating subjective assessment of a person or thing that take go when used with bí. Tá sé go maith. Tá sé go brae. Tá sé go deas. Tá sé go dona. Tá sé go haoibhinn. Tá sé go híontach. Tá sé go hálainn. Tá sé go holc. See page 16 of Learning Irish, Basic Irish Grammar and Workbook unit 16, the TEG A1 and A2 syllabuses and Buntús na Gaeilge. I have never experienced Colingo's example usages. Of course if you are using the construction with the copula then 'go' does not apply. Nach deas iad? http://www.teg.ie/_fileupload/syllabi/A2_syll.pdf http://www.teg.ie/_fileupload/syllabi/A1_syll.pdf All I can say is it's not a construction I have seen or heard used by native speakers and it's not a construction I would use myself.

But you wouldn't need the "go" when describing things as in the following examples: Cheannaigh mé cóta deas inné. Is fear deas é. :-)

I really like the Connemara dialect too. :D

Thanks for you clarification and comment on Coligno's answer. This is all very cool. :)