The calculated distance (air line) between Howth and An Muileann gCearr is approximately 53 miles respectively 86 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 Howth and An Muileann gCearr has been calculated here.
Air line (approximately)
86 kilometers
46 nautical miles