The calculated distance (air line) between Longford and An Muileann gCearr is approximately 23 miles respectively 38 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 Longford and An Muileann gCearr has been calculated here.
Air line (approximately)
38 kilometers
20 nautical miles