- Video report by ITV News Correspondent Emma Murphy
The Pope has met with eight survivors of clerical and institutional abuse on the first day of his trip to Ireland and apologised to them.
Francis held a "polite and cordial" discussion with the group for around 90 minutes on Saturday evening, focussing on the plight of past residents of Catholic homes for mothers and babies and victims of forced and illegal adoption.
The meeting took place at the Papal Nuncio's - the Pope's diplomatic representative to Ireland - residence.
A letter from survivors released afterwards said: "Around 100,000 single mothers who were forcibly separated from their babies were regularly told it was a mortal sin to search for, or even contact, their own sons and daughters.
"As an act of healing, Pope Francis, we ask that you make it clear to the now elderly and dying community of natural mothers and adoptees that there is no sin in reunion and rather that it is a joyous event that should be encouraged and facilitated by the Catholic Church."
They said many natural mothers and adoptees will be in attendance at a papal Mass on Sunday.
The letter added: "Five orders and congregations of Catholic nuns ran Ireland's notorious Mother and Baby homes where over 6,000 babies and children died as well as dozens of young mothers.
"These nuns have never taken responsibility for their wilful neglect. We ask you, Pope Francis, to publicly call on these nuns to acknowledge their actions and issue an unqualified apology to all the survivors of their institutions."
Clodagh Malone, who was born in Saint Patrick's Mother and Baby home in Dublin and adopted at 10-weeks-old, asked the Pope to publicly state that the natural mothers who lost their babies to adoption had done nothing wrong, and call for reconciliation and reunion for these families broken by the Catholic Church both in Ireland and around the world.
The Pope agreed to include the message in his Mass on Sunday, survivors said.
Francis' meeting comes after he prayed for abuse victims and expressed his "pain and shame" at the failure of church authorities to tackle the grave scandal.
While visiting a Dublin cathedral earlier on Saturday, Pope Francis sat in prayerful contemplation by a candle perpetually lit for those abused.
The gesture at St Mary's Pro Cathedral came after the pontiff acknowledged that people had a right to be outraged at the response of senior figures in the Catholic Church to the "repellent crimes" inflicted on young people.
With the reverberations of a litany of church sex abuse scandals casting a shadow over the first papal visit to Ireland in almost 40 years, Francis confronted the issue in his address inside the castle's St Patrick's Hall.
However, while he expressed regret for what had happened, ITV News Correspondent Emma Murphy said his failure to outline actions he is going to take will have upset victims.
At some point over the weekend, the Pope will meet privately with a number of abuse victims.
In his speech on Saturday, he said: "With regard to the most vulnerable, I cannot fail to acknowledge the grave scandal caused in Ireland by the abuse of young people by members of the church charged with responsibility for their protection and education.
"The failure of ecclesiastical authorities - bishops, religious superiors, priests and others - adequately to address these repellent crimes has rightly given rise to outrage and remains a source of pain and shame for the Catholic community."
He said the Church in Ireland had played a role in child welfare which could not be obscured.
"It is my hope that the gravity of the abuse scandals, which have cast light on the failings of many, will serve to emphasise the importance of the protection of minors and vulnerable adults on the part of society as a whole," he said.
"In this regard, all of us are aware of how urgent it is to provide our young people with wise guidance and sound values on their journey to maturity."
The Pope said he also wished to acknowledge women who in the past had "endured particularly difficult circumstances".
- On Saturday morning, Pope Francis arrived in Ireland and met with the country's president and taoiseach.
The speech came after a private meeting with Taoiseach Leo Varadkar, and later the Irish premier urged the Pope to "listen to the victims" of clerical abuse and called for zero-tolerance for those who target children, in his own address at Dublin Castle.
In a speech in front of the pontiff, Mr Varadkar also said the past treatment of many women and young people by church and state had left a history of "sorrow and shame".
He continued: "In place of Christian charity, forgiveness and compassion, far too often there was judgement, severity and cruelty, in particular towards women and children and those on the margins.
"Magdalene Laundries, Mother and Baby Homes, industrial schools, illegal adoptions and clerical child abuse are stains on our state, our society and also the Catholic Church."
The taoiseach called on the Pope to use his "office and influence" to "bring about justice and truth and healing for victims and survivors".
He continued that the Ireland of today is "very different" to that which saw the last papal visit, stating that it was "more diverse" and called on the Pope to help "us to build a new relationship between church and state in Ireland - a new covenant for the 21st century."
After praying for the victims of abuse at St Mary's Pro-Cathedral, Pope Francis then addressed a congregation compromising hundreds of engaged and newly-married couples, and questioned those that claimed people no longer wanted to get married.
"Getting married and sharing your lives is a beautiful thing," he told them.
He said older people had great wisdom, then joked: "Even the mothers-in-law, even they are wise."
The Pope also offered advice to couples who found themselves rowing.
"A marriage with no fights must be boring," he said.
"If even plates fly and crockery flies the secret is to make it up before the end of the day."
Following the address, Pope Francis left the cathedral and undertook a short tour of Dublin on the Popemobile.
He then arrived at the Capuchin Day Centre, which supports many of Dublin's homeless and praised the work of the centre, saying: "The church has real need of this witness, so thank you."
With Ireland in the midst of a high-profile homelessness problem, the Pope addressed a gathering of disadvantaged people who use the centre and thanked them for trusting the Capuchin fathers.
"They help you without taking away your dignity," he said.
"That's the face of Jesus Christ."
The Pope offered a blessing to those in attendance before meeting a number of volunteers who work in the centre.
Pope Francis arrived in Ireland on Saturday morning and was met off his plane at Dublin Airport by senior Irish clerics and other dignitaries who were gathered on the tarmac of the runway.
Irish and Vatican flags were flown from the cockpit windows as the aircraft taxied toward its stand.
Shortly afterwards, the pontiff met the Irish President at his official residence in Phoenix Park.
During the meeting, Michael D Higgins told the pontiff of the anger felt by those in Ireland who were abused as children by Catholic clerics.
As the Pope undertakes numerous engagements in Dublin and Co Mayo over the weekend, hundreds of thousands of people are expected to welcome him.
However, during his whistle-stop, 36 hour tour, the Pope will witness a country that has undergone seismic social changes in the four decades since the last papal visit in 1979, when John Paul II was lauded by a nation shaped by its relationship with an all-powerful Catholic Church.
- On Saturday afternoon, Pope Francis addressed young couples at St Mary's Pro-Cathedral and visited a centre for homeless people
Francis is ostensibly in Ireland to attend the World Meeting of Families (WMOF) - a major global church event focused on promoting family values, but will fulfill a number of other engagements.
More than 82,000 people attended the Croke Park concert-type event, and as well as speaking at the meeting, pilgrims also enjoyed performances by Andrea Bocelli, The Riverdance Troupe, Nathan Carter, Dana Masters and Daniel O'Donnell.
Pope Francis delivered an address in which he warned about the dangers social media poses to relationships, and heard five testimonies by families from Ireland, Canada, India, Iraq, and Africa about the importance of family in the modern age.
The pontiff warned that while social media can serve to build a network of friendships and solidarity, it should only be used in moderation, and that families should question whether they need to cut down on the time they spend using technology and instead spend more quality time with each other.
He said it was important that social media "never becomes a threat to the real life relationships by imprisoning us in a virtual reality and isolating us from the very real relationships that challenge us to grow".
On Sunday, the Pope will fly west to Co Mayo, where he will follow in the footsteps of John Paul II and take part in a religious service at a holy shrine in Knock.
He will then return to Dublin for the closing centrepiece of the WMOF event - an outdoor Mass in front of an expected congregation of half a million people in Phoenix Park.