Pope Francis has released a video message ahead of his visit to Ireland for the World Meeting of Families, saying he hopes the festival will bring "renewed encouragement to families everywhere".
He will attend a number of events in Dublin this weekend and visit Knock Shrine in Co Mayo. The pontiff is also due to meet victims of clerical sexual abuse during his visit.
In the video, Pope Francis says he is "excited" about his return to Ireland.
The pope, or Jorge Mario Bergoglio as he was then, spent three weeks with the Jesuit community in Milltown in Dublin while learning English in early 1980.
He said: "I hope that this festival will be a source of renewed encouragement to families everywhere, especially those families that will be present in Dublin.
"May it remind us all of the essential place of the family in the life of society and in the building of a better future for today’s young people."
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The World Meeting of Families officially opened at the RDS in Dublin today, ahead of the pope's visit on Saturday and Sunday.
The Archbishop of Dublin welcomed the thousands of international visitors "to celebrate and foster family life".
During the homily at the opening ceremony, 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'.
"The family is not a remote ideological notion but the place where compassion, kindness, gentleness, patience and forgiveness are learned, practised and spread."
Organisers say they are expecting 37,000 people to attend the event throughout the week.
Bells were rung this evening in Catholic cathedrals across Ireland's 26 dioceses to mark the start of the week’s events.
At St Mary & St Anne's Cathedral in Cork city representatives of the 68 parishes of the Diocese of Cloyne and Ross attended the formal Evening Prayer service to mark the official opening of the World Meeting of Families.
The prayers were led by the Bishop of Cork & Ross, Dr. John Buckley, with the homily given by Fr Aidan O'Driscoll from Clonakilty.
Symbols brought to the altar included a relic of the True Cross, a globe and a World Meeting of Families candle.
Each of the parishes in the diocese received their own special candle to burn over the coming five days until the gathering finishes on Sunday.
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Dioceses across the country brought their own personal touch to tonight's formal celebration of Evening Prayer.
In Tuam, Archbishop Michael Neary was joined by Dean Alistair Grimason for an ecumenical celebration in the Cathedral of the Assumption. While visiting pilgrims from Russia, Belarus and Poland joined the Solemn Evening Prayer at Carlow Cathedral led by Bishop Denis Nulty with Cardinal Peter Turkson as homilist.
The RDS is the venue for the meeting's Pastoral Congress. There will be three days of panel discussions with around 300 speakers.
The event will conclude with the Papal Mass of the World Meeting of Families in Phoenix Park on Sunday.
Families and pilgrimage groups will travel from as far as Africa, Canada, Europe, Australia and India for the three-day event.
However, the event has already been overshadowed by the church's handling of clerical sex abuse cases and the withdrawal of the event's keynote speaker Cardinal Donald Wuerl.
The Archbishop of Washington decided not to come to Ireland following a highly critical report in the US on the handling of abuse cases.
He will be replaced by the Primate of All Ireland Archbishop Eamon Martin.
The Vatican's key adviser on clerical sex abuse, Archbishop of Boston, Cardinal Seán P O'Malley, who was due to chair a debate on child safeguarding on Friday, also pulled out because of an alleged abuse scandal in his main seminary.
Event aims to be 'joyful family experience'
WMOF spokeswoman Brenda Drumm said that many of the visitors will be families.
"We'll have between 12,000 and 14,000 overseas visitors then the rest are from Ireland," she said.
"We will have around 6,500 people under the age of 18, and to put that into perspective, for the last Meeting of World Families in Philadelphia, the pastoral congress was attended by 17,000 people and there was 800 under-18s.
"This is a paid event, all adults paid to get here while under-18s are free."
Around 20,000 people will attend the daily mass at the RDS.
"We are covering everything from technology, to domestic abuse, human trafficking, the impact of technology on family prayer life," Ms Drumm added.
"People will also get to listen to around 44 bishops, cardinals and priests who are also speaking at the events.
"There's something here for everyone. We've been working really hard at this for three years and want to give people a joyful family experience."
Full text of the Pope's video message:
"Dear Friends,
"As I prepare to visit Ireland in a few days' time for the World Meeting of Families, I send a warm word of greeting to all the Irish people. I am excited at the thought of returning to Ireland.
"As you know, the World Meeting is a celebration of the beauty of God's plan for the family. It is also an occasion for families from all over the world to meet and support one another in living out their special vocation.
"Families today face many challenges in their efforts to embody faithful love, to bring up children with sound values and to be a leaven of goodness, love and mutual concern in the larger community. You know all this.
"I hope that this festival will be a source of renewed encouragement to families everywhere, especially those families that will be present in Dublin. May it remind us all of the essential place of the family in the life of society and in the building of a better future for today's young people.
"Young people are the future! It is very important to prepare them for the future, preparing them today, in the present, but also rooted in the past: young people and grandparents. This is very important.
"Although the specific reason for my visit to Ireland is the World Meeting of Families, I would like to include all the members of the Irish family.
"In a particular way, I pray that it may further the growth of unity and reconciliation among all Christ's followers, as a sign of that lasting peace which is God's dream for our whole human family.
"I know that many people are working hard to prepare for my Visit, and I thank all of them from my heart. I ask everyone to pray that this great festival will be a moment of joy and serenity, a caress of Jesus' tender love for all families, and indeed, for all God's children.
"I assure you of my closeness in prayer, and I would ask you to pray for me. From my heart, I send you my blessing.
"May God, Father, Son and Holy Spirit, bless you all."
Additional reporting: Jennie O'Sullivan, PA
