The calculated distance (air line) between Roscommon and Dún Laoghaire is approximately 87 miles respectively 141 kilometers.
By car or train, the actual journey to Dún Laoghaire is certainly longer, as only the direct route (as the crow flies) between Roscommon and Dún Laoghaire has been calculated here.
Air line (approximately)
141 kilometers
76 nautical miles