The calculated distance (air line) between Fermoy and Dún Laoghaire is approximately 120 miles respectively 193 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Fermoy and Dún Laoghaire has been calculated here.
Air line (approximately)
193 kilometers
104 nautical miles