"The big boy is nice."
Translation:Tá an buachaill mór go deas.
> Some subjective adjectives of assessment take the adverbial partigle go when used with the verb bí
That's from [Gramadach na Gaeilge] (http://www.nualeargais.ie/gnag/gram.htm) under the adjectives -> predicative adjectives. Deas is just one of those that requires the use of go in situations like this. The translation in the sentence box here is incorrect. Others include maith, olc, dona and iontach. They don't use go however, when they're qualified by additional adverbs.
Tá an madra beag - "the dog is small" - you are describing the dog.
is is used when you use a noun/pronoun to identify or categorize a noun/pronoun. In Is meadra beag é, you are using the noun madra to categorize the pronoun é. The adjective beag is just decoration on the noun.
is madra é - "it is a dog"
is madra beag é - "it is a small dog"
Tá an madra beag - "the dog is nice" - there is only one noun or pronoun in this sentence.