I'd say 'yesterday night' doesn't sound grammatical, in the same way that 'last morning', 'last afternoon', and 'last evening' don't either. In the same way, 'oidhche an-dè' (literally yesterday night) doesn't sound right in Gaelic, we have a specific word for it:
yesterday morning > madainn an-dè
yesterday afternoon/evening > feasgar an-dè
last night > a-raoir
Mark (2003) gives - and uses - both without any comment. Both AFB and Dwelly give only an-raoir (see faclair.com).
That suggests that both should be acceptable. When I think about it, I'm sure most people say a-raoir but I cannot say which I have observed more in writing.
There has been a general trend to dropping the n of an, regardless of meaning. I am sure it was an originally. In Irish they still write an mháthair for a' mhàthair 'the mother' and up till 100 years ago you could still see this in Gaelic, but only for c and g: an chuileag 'the fly'. This unevenness is typical of things that are changing. It is correct to write an làmh but many people say a' làmh.
I was actually thinking back to this thread today, and I was thinking whether it is right or not I believe it should be added to the right answers because someone translating to that obviously understood the sentence and it's a bit sad to lose a life for that. After all we're here to learn gaelic not english. I think being too nitpicky on what is correct or not in the english translations does no one any good