The calculated distance (air line) between Rathgar and Dún Laoghaire is approximately 6 miles respectively 10 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Rathgar and Dún Laoghaire has been calculated here.
Air line (approximately)
10 kilometers
5 nautical miles