The calculated distance (air line) between Blessington and Dún Laoghaire is approximately 19 miles respectively 30 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Blessington and Dún Laoghaire has been calculated here.
Air line (approximately)
30 kilometers
16 nautical miles