Couldn't or shouldn't this be Ta muid ag fagail e anseo?
Doesn't the way it is above mean We are to leave it here?
No. You can't have a pronoun as the direct object of a verbal noun. The structure changes when you do.
How are prepositional pronouns effected by the VB?
Are you talking about prepositional pronouns (liom, leat, etc) or pronouns in general?
Nothing. It stays the same. Only changes with pronoun direct object (if there was a prepositional pronoun, it'd be the indirect object)
Ah, I see now. Thanks.
Is “Táimid” valid to use here?
táimid and tá muid are equivalent.
Why ist this "we are leaving here" and the sentence not "Táimid é sin a fhágáil anseo" or Táimid a fhágáil é sin anseo?
It's not "we are leaving here" because of the á. And it's á because "it" is the indirect object of the progressive verb "leaving", and therefore both "Táimid é sin a fhágáil anseo" and "Táimid a fhágáil é sin anseo" are ungrammatical.
Cén fath nach bhfuill, Táimid á fhágáil ceart