Most mainline, Inter-City and suburban train services, except the DART, face disruption because of today's pay and rostering dispute between the breakaway Irish Locomotive Drivers' Association and Iarnrod Eireann.
The Munster region is likely to be worst affected, although most Cork-Cobh trains are expected to operate normally.
Iarnrod Eireann's human resources manager, Mr John Keenan, maintained yesterday that train drivers would have more leave days, as well as significantly better pay than at present, during the transition period from current rosters to new ones.
Mr Keenan said that any driver reporting for duty this morning who refused to operate the new agreement would not be permitted to work. Instead, he would be instructed to report for work on the following day. Drivers would be paid only for shifts worked, he explained.
It is not expected that the Minister for Public Enterprise, Ms O'Rourke, will make any last-minute intervention in the dispute.
No trains will operate from Dublin to Limerick, but there will be one service from Limerick to Dublin. Only three of the eight Dublin to Cork trains and two of the seven from Cork to Dublin are likely to operate. There will be one train each way between Dublin and Tralee.
About two-thirds of services between Dublin and Galway and between Dublin and Westport are expected to operate. Three out of five trains will operate between Dublin and Waterford. Most services are expected to operate normally between Dublin and Sligo, as are those between Dublin and Rosslare. The only trains not expected to operate on the Dublin-Belfast route are the 13.20 from Dublin and the 16.01 from Belfast.
Disruption is expected on the Drogheda suburban service and on the Kildare Arrow. All Maynooth services are expected to operate normally except for the 05.50 from Dublin and the 06.35 from Maynooth.
However, the actual level of disruption could be substantially less than anticipated, as Iarnrod Eireann's figures are based on all 130 ILDA members refusing to work the new rosters.
Only 76 members turned up to the extraordinary general meeting of the association in Dublin yesterday and some of these arrived after working the new rosters at the Inchicore depot.
It will be lunchtime before the scale of today's disruption becomes clear.
According to the company, 55 ILDA members, including three members of the executive, have accepted compensation for loss of overtime earnings incurred by the new rosters. About 165 drivers in SIPTU and the National Bus and Rail Union, who accepted the new agreement, are expected to work normally. However, about 30 of these are trainees.
Because the ILDA is not a recognised trade union, it has no protection from suits for damages if it engages in industrial action. Besides the company, passenger and freight customers could sue the association and its individual members for losses.
The ILDA's executive secretary, Mr Brendan Ogle, said after yesterday's meeting: "ILDA members will present themselves for their normal duties, as we have done for years." He emphasised that the way to resolve the dispute was through "dialogue and fair process".
Iarnrod Eireann has advised all intending passengers to consult its information line: 1850 366 222.