The calculated distance (air line) between Cobh and Dún Laoghaire is approximately 135 miles respectively 217 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Cobh and Dún Laoghaire has been calculated here.
Air line (approximately)
217 kilometers
117 nautical miles