The calculated distance (air line) between Westport and Dún Laoghaire is approximately 143 miles respectively 230 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Westport and Dún Laoghaire has been calculated here.
Air line (approximately)
230 kilometers
124 nautical miles