Around 500,000 people had tickets to attend the mass in Phoenix Park in Dublin.
During his address to the crowd, he described the moment he met survivors of the church’s sex abuse scandal on Saturday.
The Pontiff met with eight survivors and said he was seeking forgiveness.
He said: “We ask forgiveness for the abuses in Ireland, abuses of power, of conscience, and sexual abuses perpetrated by members with roles of responsibility in the church," he said.
"In a special way, we ask pardon for all the abuses committed in various types of institutions run by males or female religious and by other members of the church, and we ask for forgiveness for those cases of manual work that so many young women and men were subjected to.
"We ask forgiveness."
The mass was billed as one of the biggest open air events in Europe this year.
Hundreds of thousands of people waved flags and cheered as Francis entered the park on his Popemobile.
They braved a three kilometre hike and torrential rain to see the Pontiff in person.
At the same time, just a few miles away from Phoenix Park, thousands of people turned out in Dublin City centre for a protest.
The Stand for Truth rally started at the same time as the Pontiff's mass.
Some protestors were pictured with banners which read "Secular Justice for All, Truth Justice Love and Church Without Abuse."
He told the crowd there that no-one could fail to be moved by stories of young abuse victims who were “robbed of their innocence” and left with “painful memories.”
