Tá sé maith or Tá sé go maith?

I have seen both done by native speakers, but I don't understand the difference. Can anyone help?

I think it is "Tá sé go maith" in a careful speech. No "go" is needed when there is another modifier before "maith". Study these examples:

Tá sé ana-mhaith. (=very)
Tá sé ro-mhaith (=too)
Tá sé iontach maith (=very, Dún na nGall)

Le meas

THanks for your post Róman. Someone else told me today that if you say Tá sé maith, it is a condition something is in. If you say go maith, you give approval. Anyone else heard that explanation before?

Basically, "go maith" is an adverb, and "maith" is an adjective. Go maith = well ; maith = good.

Now, in Modern Irish the use of one or another depends on the dialect (you use much more "go" before predicative adjectives in Connemara than in Ulster, for example).

In Connemara, you'd say "tá sin go deas", I think. In Ulster, people would say "tá sin deas".

According to the grammar rules there are 8 adjectives indicating subjective judgement (when not qualified by an adverb) that go with the particle 'go':
go maith
go breá
go deas
go dona
go haoibhinn
go hiontach
go hálainn
go holc
I know both tá sé maith and tá sé go maith are used. I think it is basically a matter of in what part of Ireland you're in.