"Ní leanbh mo mhac ach fear."
Translation:My son is not a baby but a man.
I think the word order is different with the copula (is or ni sentences) as opposed to with bi (ta or nil sentences). It is generally VOS with the copula, I believe. For example, Is cailin me (VOS) is how you say I am a girl (SVO in English). But a sentence such as Ta an cailin fuar is VSO.
A copular statement like Is cailín mé is better described as CPS (copula, predicate, subject), since it doesn’t have an object.
"My son is not a baby but is a man"? ('is" is implied in the english translation at least)
Why is "my son" after "baby" given the normal VSO order? Wouldn't it be "Ní mo mhac leanbh ach fear"?
It’s a classificational copular statement, and the order for that is CPS (copula, predicate, subject).
Note that as given (on 2015-06-25), the Irish sentence is only valid in Ulster Irish; in the other dialects and in the Caighdeán, it should be Ní leanbh é mo mhac ach fear.