The calculated distance (air line) between Castleknock and Dún Laoghaire is approximately 11 miles respectively 18 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Castleknock and Dún Laoghaire has been calculated here.
Air line (approximately)
18 kilometers
9 nautical miles