The calculated distance (air line) between An Muileann gCearr and Castleknock is approximately 41 miles respectively 67 kilometers.
By car or train, the actual journey to Castleknock is certainly longer, as only the direct route (as the crow flies) between An Muileann gCearr and Castleknock has been calculated here.
Air line (approximately)
67 kilometers
36 nautical miles