The calculated distance (air line) between Monaghan and Dún Laoghaire is approximately 74 miles respectively 120 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Monaghan and Dún Laoghaire has been calculated here.
Air line (approximately)
120 kilometers
64 nautical miles