The calculated distance (air line) between Monkstown and An Muileann gCearr is approximately 51 miles respectively 83 kilometers.
By car or train, the actual journey to An Muileann gCearr is certainly longer, as only the direct route (as the crow flies) between Monkstown and An Muileann gCearr has been calculated here.
Air line (approximately)
83 kilometers
45 nautical miles