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