The calculated distance (air line) between Portarlington and Dún Laoghaire is approximately 45 miles respectively 72 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Portarlington and Dún Laoghaire has been calculated here.
Air line (approximately)
72 kilometers
39 nautical miles