Up to a quarter of a million extra train journeys are planned for the biggest Irish event in 40 years as people travel to see the Pope say Mass in Dublin.
Rail operator Iarnrod Eireann has laid on extra services on all main routes to the capital and advance booking is mandatory.
The papal Mass will be celebrated in Phoenix Park on the edge of the city centre on Sunday August 26 and half a million entry tickets are to be made available.
Organisers believe it will be the largest single event the country has hosted since Pope John Paul II visited in 1979.
Extra trains will operate from Cork, Kerry, Limerick, Galway, Mayo, Sligo, Rosslare and Waterford to and from Dublin.
High-frequency commuter services will ply routes near Dublin.
The rail company said intercity train tickets will not be available for sale on the day and there will be no access for anyone who does not have a ticket or reservation.
The pope will visit Ireland for two days to coincide with the World Meeting of Families.
Held every three years, this major international event brings together families from across the world to celebrate, pray and reflect upon the central importance of marriage and the family.
Customers can book travel at www.irishrail.ie or 018366222 and are advised to do so as soon as possible, as trains will sell out.
Holders of tickets to the papal Mass can travel free of charge from some commuter destinations.
An Iarnrod Eireann spokesman said: "We are looking forward to playing a major role in bringing customers to and from the World Meeting of Families events.
"The Phoenix Park Mass is the largest single event the country has hosted since Pope John Paul II visited in 1979."
There will also be special rail services from Westport, Ballina and Athlone to/from Claremorris for the Pope's Knock shrine event on August 26, including a free bus transfer.