The news came as An Garda Siochana unveiled the transport and security plans for the papal visit at Dublin Castle on Thursday.
Francis visits Dublin and Knock on the weekend of August 25 and 26 on the invitation of the World Meeting of Families 2018.
On Saturday August 25, the Pope will visit the Apostolic Nunciature on the Navan Road and the Capuchin Day Centre for the homeless in Dublin, before an evening mass in Croke Park.
On the morning of Sunday August 26, he will visit Knock Shrine and Church before heading to Phoenix Park for mass in the afternoon.
Tim Gaston, director of public transport services at the NTA, said: "Every bus, train and tram is being used for this event, and for the safety of the travelling public, if you have a ticket for the Phoenix Park, you will be allowed to travel for free within Dublin on the day of the event."
It is understood the NTA will reimburse the transport services for lost revenue on the day.
More than 500,000 people are expected to attend Dublin's Phoenix Park for the papal mass, the equivalent of seven All-Ireland football final spectator crowds.
Gardai were not willing to discuss numbers of guards being deployed for the visit, but noted there would be a "significant number" of security forces on the ground and say they are planning appropriately.
The World Meeting of Families will hold daily mass in the RDS in Dublin and expects up to 37,000 visitors per day, while the Croke Park event will see around 80,500.
After the mass, visitors are warned that from their standing position in the park to where they meet their coach or train could take up to four hours.
Both Gardai and the NTA were keen to emphasise that Dublin will be open for business over the four days, and they were working with business owners and local residents to ensure as little disruption as possible.
It is expected Monday August 25 will be the first day back at work for many people after the summer break. However, Garda Superintendent Thomas Murphy said his main priority is the safety of all visitors.
"This is the largest event Ireland has organised in nearly 40 years.
"I hope to have everyone home safely by that stage and I'm sure the local authorities will have the place clean and ready to go on Monday morning.
"We are urging people to go by train, bus, coach or Luas to this event. While walking is inevitable, you will be minimising your walk to and from the event by taking public transport or private coaches."
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