"He is a positive child."
Translation:Is páiste dearfach é.
No. The reason that you use is is because you are linking two nouns "he" and "child" (actually a pronoun and a noun).
There is only one noun in "the child is positive" so you don't need the copula.
"X is (a/the) Y" uses the copula if Y is a noun.
"X is Y" doesn't use the copula if Y is an adjective.
(For the sake of clarity, it's not just "X is (a/the) Y" - if X is "I", then it would be "X am (a/the) Y", if X is "they" then it would be "X are (a/the) Y", etc).
That isn't really it. Is is used to ling two nouns. Period. It doesn't matter what the emphasis is. If I say "Tá an turtar marbh", isn't turtar the main concern? Yet I use Tá. This is NOT comparable to the Spanish ser and tar! It is simply used to link 2 nouns. What you're saying does seem true because often you're using an adjective instead of a noun after the first noun. But it really is that simple : the copula is used when you're linking or comparing 2 nouns.