The calculated distance (air line) between Lucan and Dún Laoghaire is approximately 14 miles respectively 22 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Lucan and Dún Laoghaire has been calculated here.
Air line (approximately)
22 kilometers
12 nautical miles