Irish Rail says a DART extension that was granted planning permission today will "transform" communities.
Dart+ South West adds an electrified track through Park West, Cherry Orchard, Clondalkin, Lucan and Celbridge.
The project includes adding track and re-signalling the line, building a new station at Heuston West and upgrading the Phoenix Park Tunnel.
Irish Rail says the process of finding a construction contractor will follow - it hopes to break ground in 2026.