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