A crowd of more than 12,500 pilgrims from "all four corners of the world" gathered in the RDS yesterday for the opening ceremony.
A spokeswoman for the Congress said they expected numbers to increase during the week, with up to 80,000 expected at Croke Park next Sunday.
"We had just over 22,000 e-tickets booked for Congress, some of them were seven day tickets, some three day and a small number of one day tickets," she said.
Some of those with seven day tickets would not be arriving until today.
"We still had more than double the last Congress in Quebec (four years ago)."
The last International Eucharist Congress held in Dublin, in 1932, attracted more than one million people, a quarter of the population.
"The numbers are certainly nowhere near the 1932 numbers but the turnout is considerably higher than you had in Quebec for the 49th Congress four years ago," the spokeswoman said.
"We expect the numbers will increase as the week goes on."
Asked if the good weather or the Ireland Euro 2012 clash might have had something to do with the attendance, she said: "I don't know. The numbers are very good and they will increase during the week."
A ceremony celebrating Irish Catholic heritage opened the Congress in Dublin yesterday. The event is expected to attract between 10,000 to 12,000 each day over its eight day run.
Archbishop of Dublin Dr Diarmuid Martin and the Papal Legale Cardinal Marc Quellet addressed an audience made up largely of overseas pilgrims.
Speaking during the ceremony, Dr Martin said the Church in Ireland was on a path to renewal.
He called on thousands of pilgrims to remember victims of clerical abuse.
"The 50 years since the Second Vatical Council have brought many graces to the Church in Ireland," he said. "But those 50 years have also been marked with sinful and criminal abuse and neglect of those weakest in society: children who should have been the object of the greatest care and support."