The calculated distance (air line) between Castlebar and Dún Laoghaire is approximately 135 miles respectively 218 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Castlebar and Dún Laoghaire has been calculated here.
Air line (approximately)
218 kilometers
118 nautical miles