The calculated distance (air line) between Birr and Dún Laoghaire is approximately 75 miles respectively 121 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Birr and Dún Laoghaire has been calculated here.
Air line (approximately)
121 kilometers
65 nautical miles