The calculated distance (air line) between An Muileann gCearr and Rathfarnham is approximately 46 miles respectively 74 kilometers.
By car or train, the actual journey to Rathfarnham is certainly longer, as only the direct route (as the crow flies) between An Muileann gCearr and Rathfarnham has been calculated here.
Air line (approximately)
74 kilometers
40 nautical miles