Pope Francis hopes to begin his historic visit to Ireland in Knock, the Herald understands.
Organisers of the Pontiff's trip are believed to be exploring the idea of having him fly directly to the west of Ireland.
There has been much speculation about the type of welcome the Pope will receive - but sources say gardai have concerns that as many as 650,000 people will want to see him in the Phoenix Park on August 26.
By holding another event at the Marian Shrine in Co Mayo a day earlier, they hope to offer an alternative viewing opportunity for people.
"The Phoenix Park can cope with the crowds once they are inside but getting them safely to and from the Mass would be the problem," a source said.
The Vatican tends to remain tight-lipped about the Pope's itinerary until shortly before he travels to a country.
It has been confirmed that the first Papal visit in nearly 40 years will coincide with the World Meeting of Families in Dublin from August 21 to 26.
Pope Francis (81) is scheduled to take part in a ceremony in Croke Park on August 25 before celebrating Mass in Phoenix Park the following day.
Well-placed sources involved in planning for the visit have indicated to the Herald that Knock is now firmly on the agenda.
One proposal is for the Pontiff to fly directly to Knock Airport, which in itself would be an historic occasion.
It was the determination of Monsignor James Horan that ensured the airport was built.
It opened in 1985 with three chartered flights to Rome and has since welcomed more than 10 million passengers. Pope John Paul II celebrated Mass at Knock Shrine during his visit in 1979.
Pope Francis has prayed at a number of Marian shrines while on tours to other countries, including last year when he honoured two children who saw Mary in the Portuguese town of Fatima. It is likely that if the Pope flies directly to Knock, he will then use air transport to get to Dublin in time for a ticketed event in Croke Park.
His itinerary in the capital is also expected to include a visit to the Capuchin Day Centre run by Br Kevin Crowley.
The Archbishop of Dublin, Diarmuid Martin, has also confirmed that the Pope wants to visit a prison during his time here, most likely Mountjoy.
The trip is expected to cost in the region of €20m, with the costs being picked up by the Catholic Church in Ireland and the Irish taxpayer.