Pope Francis has expressed hope his visit to the Republic of Ireland will "further the growth of unity and reconciliation" among all Christians.

In a video message to mark the start of the World Meeting of Families, he said he was "excited at the thought of returning to Ireland".

Pope Francis' 36-hour visit begins on Saturday and coincides with the global gathering of the Catholic Church.

Catholic cathedrals marked the launch by ringing bells on Tuesday evening.

The World Meeting of Families is held every three years, with thousands of people from across the world attending.

It is the first time the event has been held in Ireland.

'Irish family'
In his message, the pontiff sent a "warm word of greeting to all the Irish people".

"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," he said.

"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."

The video was released hours after it was confirmed that the Pope would meet victims of clerical sex abuse during his Irish visit.

The Vatican said the meeting would not be announced until after it was over and that it would be up to victims if they wanted to speak afterwards.

The head of the Catholic Church has condemned the "atrocities" of child sex abuse by clerics and cover-ups in the Church.

He published a letter to the world's 1.2 billion Catholics on Monday, addressing failures to deal with abuse and calling for an end to the "culture of death".

Ireland is gearing up for its first papal visit in 40 years during which at least half a million people are expected to attend a special Mass in Dublin's Phoenix Park.

The Pope's 36-hour visit will begin on Saturday morning and his itinerary also includes the Festival of Families concert in Dublin's Croke Park stadium and a trip to the Knock holy shrine in County Mayo.

It is the second time Pope Francis has been to the country.

As Father Jorge Bergoglio, he visited the Jesuit community at Milltown Park in Dublin in 1980.

Meanwhile, Sinn Féin deputy leader Michelle O'Neill has said she will no longer be attending an address in Dublin Castle or the Phoenix Park Mass due to her "broken leg and restricted mobility".

She said the Pope's visit was a "significant event for all of the Irish people" and that she would be listening "with acute interest" to what he had to say on a range of issues.

"We must specifically acknowledge the damage done by the Catholic Church to the lives of many women and children in the mother and baby homes, the Magdalene Laundries and a succession of abuse scandals and cover-ups," she said.

"The visit of Pope Francis is an opportunity to address these issues, to meet with victims and survivors, to fully acknowledge their suffering and the damage done, to end the cover-ups and to commit to full redress, north and south."

Last week, Democratic Unionist Party leader Arlene Foster said she would be away with her family on holiday during the papal visit.

The party is not planning to send anyone in her place, meaning it will be the only one of the five main political parties in Northern Ireland not represented at any of the events.