Hi
The 1st edition of the 1:2500 OS shows both Mynachlog and Rhiwbryfdir House (originally Ivy Cottage) quite separately, the otiginal Rhiwbryfdir House having been buried by the tip with Mynachlog labelled on the west side of the river just at the point the Barlwyd river was bridged by both the FR and the LNWR.
Earlier quarry plans show Rhiwbryfdir house as roughly halfway between Ivy Cottage and Mynachlog, close by the foot of the earlier incline up to Rhiwbryfdir Quarry.
It is almost out of shot behind the trees on the left middle distance of the John Thomas photo on Ffestipedia, with Ivy house almost invisible under its cloak of ivy.
The 1838 WSCo. lease plan does not label Mynachlog, but does show and label Rhiwbryfdir house. The plans of the proposed route of Holland's incline show and label "Rhiwbryfdir House and Cottages" but nothing else. Again, James Spooner's 1826 map of the area labels Rhiwbryfdir, but nothing else, as does the 1818 preliminary OS survey.
If the location of Mynachlog on the 1:2500 OS is correct, then it corresponds to a small area (rather than a specific building) close by the point where the track to Rhiwbryfdir crossed the Barlwyd and was probably not significant enough to be labelled by earlier map makers, there are several small buildings in the area which could be candidates if one was wanted, but I doubt at this distance in time one can be positive. All disappeared beneath the tips long ago. It was thought something would appear when they cleared the Penybont tip - but any structures buried by it had reverted to indistinguishable slate waste!
Grahami
The map is the territory - especially in chain scale.