Iarnród Éireann has said rail passengers are facing three months of disruption to services on the main Dublin to Belfast line after part of it collapsed.
A 20-metre section of the viaduct over the Malahide estuary collapsed into the water at about 6.30pm yesterday evening.
The company has described the incident as being close to a "very serious tragedy" and a major investigation has been launched.
Bus transfers are being arranged for passengers on the northern commuter lines.
Iarnród Éireann spokesperson Barry Kenny said the alternative arrangements will be finalised before people go back to work on Monday.