The section of bridge on the main Dublin to Belfast railway line at Malahide that collapsed on Friday

There are fears that rail services in north Dublin and Louth will be affected for longer than the estimated three months after Friday's bridge collapse in Dublin. A similar accident in 2003 took 11 months to repair.

Irish Rail anticipates it will take a minimum of three months to repair the damage caused by the collapse of a section of bridge on the main Dublin to Belfast line at Malahide last Friday.

During the last bridge collapse on the rail network, at Cahir in 2003, it took 11 months before the line was eventually reopened.

The last major work on the collapsed Broadmeadow Estuary Bridge was in February and March of 2004, when rock and concrete was added to embankments at the pier, it is understood.

Keith Farrelly, the driver of a train which passed over the railway bridge just minutes before it collapsed two days ago, said: "The first inkling I had that something was wrong was when I noticed water splashing up to a high level. In that location, it's not a normal thing to happen, so I looked at the northbound line and saw that the viaduct was giving way and that the track was hanging.

"Immediately then, I could feel the ballast moving on the line beneath my train and realised the danger we were in," added the 33 year-old Dubliner based in Connolly Station. "It was such an unreal sight, I started thinking to myself 'did I really see that?', but when I walked back I saw it clearly, and my legs just went to jelly with the shock. I'm just glad that all of us on board, the passengers and myself, walked away from it safely."

There were serious disruptions yesterday for passengers on the busy Northern Commuter Line and the Belfast Enterprise service.

A shuttle service was in operation between Drogheda and Skerries while passengers travelling from the north of Dublin were advised to make other arrangements. Passengers on the Belfast Enterprise were being transported by train as far as Drogheda, where a bus transfer was available to Dublin.

A statement from Irish Rail said: "An investigation into this extremely serious incident is under way."

It said a structural examination of the bridge had taken place as recently as last Tuesday and no problems were identified.

Irish Rail said the incident had the potential to be a serious tragedy and that a full investigation would now be taking place into what had caused the collapse.

The 18.07 Balbriggan to Connolly Station train was crossing the bridge at around 6.20pm on Friday with 50 passengers aboard when Farrelly noticed subsidence on the rail line opposite.

He immediately reported what he had seen and all services were immediately stopped. Within minutes, a 20-metre section of the viaduct had collapsed.

Rail Users Ireland said the incident raised "serious questions of Irish Rail's maintenance and inspection regime". It also called for refunds for weekly, monthly, and annual ticket-holders who will not be able to travel as a result.

The final cost would be likely to be "several million euros", according to Irish Rail spokesman Barry Kenny

"Obviously, the first priority is assessing what needs to be reinstated and there would be a large cost there. There will be a loss of business and costs in terms of bus arrangements. Certainly, it will be several million euros," he said.

"There is a significant tidal flow in the area and there was quite a strong tide on Friday. We will investigate every aspect and we are not going to prejudge anything."