The announcement was made near the end of a huge outdoor Mass celebrated in front of hundreds of thousands of people in Philadelphia.
Pope Francis has just attended this year's World Meeting of Families in the city.
If he attends the 2018 event, he would be the first papal visit to Ireland since John Paul II's three-day tour in 1979, the first visit to Ireland by any pontiff.
Archbishop Eamon Martin
The Catholic Church in Ireland has been rocked by two decades of sex abuse scandals but still remains a key part of society there.
About 85% of the 4.6 million people in the Republic of Ireland are Catholics, the overwhelming majority of state-funded schools and hospitals are run by church authorities and Ireland maintains the strictest ban on abortion in Europe.
State TV and radio tolls the Angelus call to prayer twice daily at noon and 6pm.
The World Meeting of Families is an event that happens every three years and was started by John Paul II in 1994.
The Dublin conference will be the ninth.
It is widely expected a Pope would attend a meeting but pontiffs have skipped them twice.
"I am delighted to hear that Pope Francis has announced that the 9th World Meeting of Families will take place in Dublin, and that Archbishop Diarmuid Martin is in Philadelphia with our delegation to hear the news directly from the Holy Father," said Archbishop Eamon Martin, the Catholic primate for Ireland.
"Three years ago the 50th International Eucharistic Congress was a great celebration of faith for Ireland, and it attracted pilgrims from all around the world. I am confident that the World Meeting of Families in 2018 will also be an uplifting event for all of us."