The calculated distance (air line) between Buncrana and Dún Laoghaire is approximately 138 miles respectively 222 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Buncrana and Dún Laoghaire has been calculated here.
Air line (approximately)
222 kilometers
120 nautical miles