The calculated distance (air line) between Ballina and Dún Laoghaire is approximately 136 miles respectively 219 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Ballina and Dún Laoghaire has been calculated here.
Air line (approximately)
219 kilometers
118 nautical miles