The calculated distance (air line) between Letterkenny and Dún Laoghaire is approximately 131 miles respectively 212 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Letterkenny and Dún Laoghaire has been calculated here.
Air line (approximately)
212 kilometers
114 nautical miles