The calculated distance (air line) between Waterford and Dún Laoghaire is approximately 82 miles respectively 133 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Waterford and Dún Laoghaire has been calculated here.
Air line (approximately)
133 kilometers
72 nautical miles