I have seen "Nach eil e dà uair fhathast" for "Isn't it two o'clock yet", but this sentence does not use "Nach eil". It seems that "Nach e dà uair fhathast" is not correct - what is the key difference between these two sentences where you can use "Nach e" in one but must use "Nach eil e" in the other?
The two "e" are different ... in Nach e, it's a form of one of the "to be" verbs - 'S e / An e / Chan e / Nach e, which are often followed by (subject of sentence) a th' ann = There is a xxxx (See somewhere in the Tips, can't remember exactly where it is explained, could be in this skill.) In the other phrase, e = it or he/him, and is a 3rd person subject of the other form of the verb "to be" - Tha e / A bheil e / Chan eil e / Nach eil e, used, among other things, for time-telling, describing. Some situations require the "e" version of the verb and others the "tha" version - and I haven't reliably sussed out which is which myself yet, but if you can find that bit of the tips it should help. Good luck.
That is right as far as which version to use is concerned, but e is not a verb. E means 'it' or 'him' and translates the it in 'Isn't it an old building that is in it'.
The verb is either within the nach or it has gone missing - scholars cannot agree which - but in practice, the nach means 'isn't' in this sentence.
They're certainly similar, but not quite the same - two things - togalach = building; an togalach = the building, and there's no "an" here; second, sean is an attributive (right beside the noun) adjective in the example (isn't it an old building), but not in "isn't the building old", there it's predicative, if you start from the basic statement - the building is old (verb in the middle). This form stands, even when you make it into a negative or a question. And I still get these wrong with monotonous regularity. :(