Irem is wedged right up against Parabola, in a sort of metaphysical way, which is why you can enter Parabola from Irem in Sunless Sea. As such, it shows not just what Is-Not, but also what Could-Be, hence the futures. That's just a small part of the explanation, Irem is also simultaneously in the future and the past and the present so it can show what could happen and what has happened. Other than vague stuff the Devils did there isn't really an explanation for that part.