The quarry is located within the estate of Overtoun House, and may have supplied some of the stone used to build the house. The house is built of a hard sandstone called Spout of Ballagan Sandstone, a material that is found in the area.
To access the quarry I would suggest leaving the path at N55 57.646 W004 30.963, follow the path to the metal post and onto the quarry from there.
Take care if you stray into the tree planting area – the trees are planted on small mounds which helps them grow quickly when they are young – but mounds leave holes and the ground can be quite uneven.