Irish Rail is advising those travelling to Dublin for the Pope's visit next month that train seats must be booked in advance.
There will be no access to trains on Sunday 26th August without a ticket or reservation.
Irish Rail says train tickets will not be available for sale on Sunday 26th August, and only passengers with tickets or reservations will be allowed access the Intercity trains.
This applies to regular services and extra trains which will operate on a number of routes, including the Mayo, Galway and Sligo rail lines.
Train passengers must travel on the train they book - due to high demand, there is no flexibilty to change to other services.
Trains on that date will have unallocated seating, and passengers should take any available seat.
Up to half a million people will travel to Phoenix Park where the Pope will celebrate Mass on 26th August, following a brief visit to Knock Shrine.
Irish Rail says this is the large single event the country has hosted since Pope John Paul II visited in 1979, and customers need to book intercity trains in advance to ensure all can travel to the event in comfort.
Iarnrod Eireann says there are also limited services from Westport, Ballina and Athlone to Claremorris for the Knock Shrine event on the same date - including a bus transfer to and from Knock.