If fhoirgnimh is the genitive of foirgneamh why would the top of the building be incorrect. Admittedly nit picky, but I fear I am missing something obvious.
Is thuas not above. Or is that just hiberno english
Thuas can be either “above” or “up”.
"Thuas" is where you are when you reach the top. The "h" is not lenition, it is part of the word. The opposite meaning "thíos" is also a place, namely "down below".
Would it make sense to say Tá sé ag obair ar bharr an fhoirgnimh? E.g. in English, you can say "he is working on top of the building" without needing to add 'up'.