"He is a Spanish boy."
Translation:Is buachaill Spáinneach é.
It would be incorrect because that's not a grammatically valid sentence in Irish.
The TSF error is one of the most basic errors that learners make in Irish. Learners try to say "tá sé fear" , but a sentence of the form
(pro)noun == noun
is a copula, and you don't use the verb bí to say that in Irish, you use the copula is - is fear é.
This sentence is a copula: he (pronoun) == Spanish boy (noun).
Is buachaill Spáinneach é.
(There are more advanced structures that use the verb bí with the preposition i and a possessive adjective for certain copular statements, but they don't apply in this case).