The calculated distance (air line) between Ballyfermot and Droichead Nua is approximately 21 miles respectively 34 kilometers.
By car or train, the actual journey to Droichead Nua is certainly longer, as only the direct route (as the crow flies) between Ballyfermot and Droichead Nua has been calculated here.
Air line (approximately)
34 kilometers
19 nautical miles