"Is it your brush or my brush?"
Translation:An é do scuab nó mo scuab é?
Is "an í do scuab nó mo scuab í" correct given that "scuab" is baininscneach?
It's a funny copula thing. The general rule-of-thumb is that the copula (is, an, ba, etc.) cannot be directly followed by a proper noun (e.g. mo scuab, do scuab). You insert an "extra" copy of the pronoun just to prevent that situation, but this pronoun is not translated. The pronoun that translates to "it" is the one at the end.
One of the many joys of Irish grammar.
No. It's there because if you have a definite noun phrase in the copula it requires a pronoun before it.
An corresponds to "is" here.