The calculated distance (air line) between Blanchardstown and Dún Laoghaire is approximately 12 miles respectively 19 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Blanchardstown and Dún Laoghaire has been calculated here.
Air line (approximately)
19 kilometers
10 nautical miles