The calculated distance (air line) between Roscrea and Dún Laoghaire is approximately 73 miles respectively 118 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Roscrea and Dún Laoghaire has been calculated here.
Air line (approximately)
118 kilometers
63 nautical miles