"We are leaving it here."
Translation:Tá muid á fhágáil anseo.
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.
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)