The rush hour incident took place near Dun Laoghaire, leaving passengers stuck onboard for over an hour from about 6.15pm.
“We have two people who are feeling unwell, and emergency services are with them at present”, said Irish Rail spokesperson Jane Cregan.
The spokeswoman confirmed the service would not be back up and running any time soon: “We now have no services between Dun Laoghaire and Lansdowne and we don’t expect to have them back for some time”.
The train is believed to have gone off the tracks at about 6pm, after a signal fault, and Irish Rail have confirmed that a low-speed derailment has led to no services between Lansdowne Rd and Dun Laoghaire.
Dublin Bus is also honouring rail tickets during the disruption.
Irish Rail described the derailment as “low speed” and said there were no reports of injuries. Passengers using the Rosslare service were told they would have to avail of bus transfers.
“Iarnród Éireann, The Rail Accident Investigation Unit and the Commission for Rail Regulation will all be investigating this incident”. As a result, services have been suspended for the time being.
Customers onboard the train affected were walked back to the nearest station by Iarnród Éireann staff.