The calculated distance (air line) between Rathgar and An Muileann gCearr is approximately 46 miles respectively 74 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 Rathgar and An Muileann gCearr has been calculated here.
Air line (approximately)
74 kilometers
40 nautical miles