Religious and political leaders in Ireland have paid tribute to Pope Benedict XVI, following his shock resignation.
Cardinal Sean Brady the pope's decision had been a "profound act of humility".
NI deputy first minister Martin McGuinness said everyone was surprised.
In an address in Armagh, Cardinal Brady, the head of Ireland's Catholics, said it was an "historic day".
"With typical humility, courage and love for the church he has clearly come to the view that the Lord now wants him to use his remaining physical and spiritual energies by serving the church in prayer. I think this is a profound act of humility, a conscientious and responsible decision to hand over the ministry of the successor of St Peter in a time of great challenge for the a church and for faith in the modern world."
He added: "On behalf of the Catholic Church in Ireland, I thank him for his generous service to the church and for the great love and concern he has always shown to the church in Ireland."
Cardinal Brady said that Pope Benedict XVI was the "man who had the courage" to make such a decision.
Northern Ireland deputy first minister Martin McGuinness said: "Interest understandably centres on his health and the deteriorating nature of it. The resignation of a Pope is a most unusual occurrence and this clearly suggests that Pope Benedict's has such very serious concern about his health that he feels he must resign. The thoughts and prayers of everyone will be for Pope Benedict. I hope that the lifting of the onerous weight of the responsibilities of such an important world leader will ease the burden on him in his retirement."
Archbishop of Dublin Diarmuid Martin has said that he believed that history would look at the Pope in a "varied way".
Dr Martin said that Pope Benedict had a very clear understanding of some of the moral problems in the church and he had addressed them "head on".
"He was a man who wanted to know a lot about what was happening in Irish society, not in a way of pointing fingers, but of trying to learn and asking me and the Irish church where are we in today's society." he said.
The archbishop said Pope Benedict was a private person and that he has a "great personal affection" for him. He said the Pope was "happiest when he was writing".
Taoiseach (Irish prime minister) Enda Kenny, who criticised the Vatican for its response to the abuse scandals in Ireland, wished Pope Benedict well and said he had given "strong leadership" to the church.
"This is clearly a decision which the holy father has taken following careful consideration and deep prayer and reflection," he said. "It reflects his profound sense of duty to the church, and also his deep appreciation of the unique pressures of spiritual leadership in the modern world."
Irish deputy prime minister (Tanaiste), Eamon Gilmore, said that like most people in Ireland his immediate concern was for the Pope's health.
Mr Gilmore wished him well in his retirement, describing him as someone who had made a huge contribution to working towards world peace.
One of the Pope's former students, Fr Vincent Toomey, has said the Pope did not look well the last time he saw him.
Speaking on Irish state broadcaster Rates' News at One, Fr Toomey, the Professor Emeritus of Moral Theology at NUI Maynooth, said he was shocked at the Pope's resignation, but in hindsight he had given "some signals" in an interview last year.
He also said that at a gathering of the Pope's former doctoral students last summer, Fr Twomey said the Pope was very alert, but very "tired-looking".
Fr Twomey said: "We all felt he looked grey and tired, and shrivelled, to a certain extent. Then he came the following day and said Mass for us and then joined us for breakfast. After breakfast he did something rather strange, which he never did before. He got up, as we thought, to leave, but he came over and he said: 'No, no, I just want to greet each one of you'.
"And I thought at the time, did he want to say goodbye? It looks like he did."
Speaking at St. Oliver Plunkett church in Lenadoon, west Belfast, Father Martin Magill said the news "came completely out of the blue, I had no idea".
"Someone sent me an email and I read it several times" he said.
"It's a very humble statement," he added. "It's very much an acknowledgment of the huge responsibility that the role comes with".
Outside St Paul's church on the Falls Road in Belfast, parishioners spoke of their shock at
Several Mass goers said that while they will be sorry to see the Pope go, they hope to see a younger person take over.
Margaret McGuckin who set up Survivors and Victims of Institutional Abuse in 2009, said: "Saying worry is a wonderful thing, but it's actions that were needed and they (the Vatican) failed to act. They set up safeguarding, but still it happens and still occurs."
She said the next pope needed to be "squeaky clean" because the church was falling apart.
The unexpected development - the first papal resignation in nearly 600 years - surprised governments, Vatican-watchers and even his closest aides.
The then Cardinal Joseph Ratzinger became Pope after John Paul II's death.
The Vatican says it expects a new Pope to be elected before Easter.