The calculated distance (air line) between Carrickmacross and Rathfarnham is approximately 50 miles respectively 80 kilometers.
By car or train, the actual journey to Rathfarnham is certainly longer, as only the direct route (as the crow flies) between Carrickmacross and Rathfarnham has been calculated here.
Air line (approximately)
80 kilometers
43 nautical miles