Dart services from Pearse Street station to Dun Laoghaire have been suspended for the foreseeable future due to flood damage to a bridge.
Ianród Éireann said it did not known exactly how long services would be disrupted but said it will be for "a number of days at least".
Rosslare and Gorey services are also affected by the disruption and will only operate to Bray with bus transfers onwards to Connolly station.
Other commuter services which operate from Drogheda or Maynooth to Dun Laoaghaire and Bray will now stop at Pearse Street station.
Dublin Bus will honour tickets for customers travelling south of Pearse Street.
The line has been closed due to damage to the ground in the vicinity of a pier on the Dodder Bridge at the south end of Lansdowne Road Station.
This area was flooded on Monday, during the torrential rainfall in the Dublin area. At least two large trees which fell were swept downstream by the River Dodder became lodged under the bridge, causing debris to build up under it.
An inspection of the single-span structure by divers tonight revealed damage to the ground in the vicinity of a support pier of the bridge. No such damage was found during anunderwater inspection of the bridge in October 2010.
Iarnród Éireann said it had closed the structure as a precautionary measure to allow repair works to be carried out.
"A remedial works programme to repair the flood damage is now being devised, and this will confirm timescales for this disruption," it said.
