Thousands descended on Dublin's Royal Dublin Society (RDS) on Tuesday evening to attend the opening ceremony.
Church bells were rung simultaneously across Ireland's 26 dioceses to launch the five-day Catholic congress. Prayers were also said throughout the dioceses.
More than 37,000 people from 116 different countries are expected to attend the RDS for a series of events as part of the Catholic festival.
Hundreds of thousands more will attend the celebrations that will be led by Pope Francis this weekend in Dublin and Knock.
The WMOF will culminate with the closing papal mass in the Phoenix Park on Sunday.
The event has gained huge significance since the Pope made a statement on Monday apologising for the atrocities of clerical sexual abuse.
In a letter to the world's more than 1 billion Catholics, Pope Francis said the Church needed to move to end the culture of death within it.
The opening ceremony, which was entitled Le Cheile le Criost, or Together with Christ, included hymns, psalms and prayers for the entire human family before God.
In his homily, the Archbishop of Dublin welcomed the thousands of international visitors to the country.
Families and pilgrimage groups will travel from as far as Africa, Canada, Europe, Australia and India to partake.
Diarmuid Martin said: "There are those who would look at the World Meeting as some sort of ideological gathering to celebrate a type of family which probably does not exist.
"Whatever of the past, here in Dublin the World Meeting is something much more profound: it is to reflect the opening words of our reading: 'You are God's chosen race; he loves you'."
Dr Martin added: "The family is not a remote ideological notion but the place where compassion, kindness, gentleness, patience and forgiveness are learned, practised and spread."
The archbishop said family life has changed in Ireland.
"It may seem a strange thing to say, but we have to find ways of ensuring that these new relationships and challenges in family culture become 'clothed in love'," Dr Martin said.
"Only the power of love can purify and restore our Church and us and our society."
He added: "We thank God for the at times immense sacrifice that exists in the love within families.
"We pray for those who have never experienced such love or from whom such love was stolen through abuse or neglect."
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