The calculated distance (air line) between Portlaoise and Dún Laoghaire is approximately 51 miles respectively 83 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Portlaoise and Dún Laoghaire has been calculated here.
Air line (approximately)
83 kilometers
45 nautical miles