In another iteration of this sentence, the translation is shown as "Sin an bhrionglóid". Are both forms correct, because I typed this response in that sentence and had it marked wrong.
I like this colloquialism; is it considered "incorrect" by language teachers for not having a verb?
No, not incorrect. The copula is implied. It's akin to that is versus that's in a lot of respects.
So was there originally a copula there and it's just merged/omitted with "sin" and "seo"?
You can say either : Is é sin an bhríonglóid or Sin é an bhríonglóid...
Though I wonder if í wouldn't be better.
The reverse translation exercise doesn't accept "sin é an bhrionglóid" as a valid translation of "That is the dream"
má tá tú ag brionglóideach faoi charr, nó faoi thuras, nó faoi phost nua, bíonn "é" ceart.