Iarnród Éireann said the “major disruption” is impacting Northside Dart, Northern Commuter and Enterprise Services.

Dart services are operating between Clontarf and Bray/Greystones, but none are running from Howth/Malahide.

There are no Northern Commuter services between Howth Junction and Connolly.

It comes amid a status yellow high temperature warning for the country, with maximum temperatures in Dublin projected to reach 24C on Wednesday.

Iarnród Éireann spokesman Barry Kenny said there would be bus transfers between Connolly and Drogheda for impacted Enterprise/Belfast services, while Leap cards are also valid on other Transport for Ireland services for impacted Dart customers.

He said: “We apologise to everybody who is affected by this.”

Asked on RTÉ’s Morning Ireland if there would be enough buses to bring people to their destinations or whether commuters should make other arrangements, Mr Kenny said: “I think people who can make alternative arrangements would be advised to do so.

“Obviously, potentially as the morning continues, this would affect between 15,000-20,000 commuters, and that’s obviously a hugely significant volume of people potentially deferring to other modes.

“As I say, it does arise from overhead line damage, and our crews are working on site to repair, but we do expect that this will be the entirety of the morning we will have this disruption.”

Mr Kenny said Iarnród Éireann was “very confident” services would be resolved before a concert in Malahide on Wednesday evening, adding: “We are hoping to be back around lunchtime.”

He explained that the incident happened before 8.30pm on Tuesday, but Irish Rail deferred repairs into the early hours of the morning as there were 20,000 people at a gig in Malahide.

He said, “I think it was the right decision to defer the start of the repair works, which would have shut the entire line.

“So we had one track open to get those people home from Malahide.

“I think potentially we would have had public safety issues if we weren’t able to clear and help move thousands of people from Malahide after the concert last night.”