Final preparations are underway ahead of the visit of Pope Francis to Ireland.
Measures are being put in place across areas such as transport, security and health.
More than 1,000 doctors and nurses will be on hand to help, while there is also a mortuary set up.
There will be 1,000 volunteers and 3,000 people in the choir, while 31,000 litres of milk and 25,000 sliced pans will be used to look after the food and drink needs of people.
Half a million people are expected to be there for the closing mass and there will be extra services across public transport for the weekend.
Margaret Gormley from the Office of Public Works said: "It's actually the largest event taking place in Europe this year. It's the equivalent of six Croke Parks on All-Ireland final day."
People heading to see the Pope in the Phoenix Park are being advised to book their train tickets quickly as some rail services are already sold out.
Tickets for the Tralee return service are completely gone, as are the morning tickets from Waterford.
There is only limited availability left on the Dublin-Waterford train and for the services from Cork and Limerick.
Advance booking is mandatory for all Intercity trains and no tickets will be available on the day.
Irish Rail customers can book travel at www.irishrail.ie or phone 01-8366222 .
For existing ticket holders, including free travel pass holders and season ticket holders, an advance reservation is essential for travel on Sunday, August 26. Reservations for this date are free of charge, and can also be booked at www.irishrail.ie and will not be available at stations.
Extra DART and Commuter services will operate that Sunday also before and after the Papal Mass.
The National Transport Authority has announced that holders of tickets to the Papal Mass travelling in the Short Hop Zone (DART, and between city centre and Balbriggan, M3 Parkway, Kilcock, Sallins, Kilcoole) can avail of free public transport on this date.
An Iarnród Éireann spokesperson said: “We cannot stress enough how essential it is for customers to book Intercity travel in advance - this will ensure that all can travel to and from the event in comfort.
"In addition, with Dublin area road restrictions, our extensive DART and Commuter service together with free travel in the Dublin area for those with Papal Mass tickets mean you can travel to access points like Connolly, Ashtown, Navan Road Parkway and Heuston conveniently.”
There are also special services from Westport, Ballina and Athlone to/from Claremorris for the Knock shrine event on Sunday, August 26, including a free Bus Éireann bus transfer to and from Knock.
As with Intercity services to/from Dublin, anyone who wishes to travel on these services must pre-book their rail ticket as there will be no ticket sales on day.
If you hold an existing ticket or are a free travel pass holder, you must reserve your seat in advance of the event.