The calculated distance (air line) between An Muileann gCearr and Letterkenny is approximately 100 miles respectively 161 kilometers.
By car or train, the actual journey to Letterkenny is certainly longer, as only the direct route (as the crow flies) between An Muileann gCearr and Letterkenny has been calculated here.
Air line (approximately)
161 kilometers
87 nautical miles