The calculated distance (air line) between Portmarnock and Dún Laoghaire is approximately 9 miles respectively 14 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Portmarnock and Dún Laoghaire has been calculated here.
Air line (approximately)
14 kilometers
8 nautical miles