The calculated distance (air line) between Castletroy and Dún Laoghaire is approximately 109 miles respectively 176 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Castletroy and Dún Laoghaire has been calculated here.
Air line (approximately)
176 kilometers
95 nautical miles