President Michael D Higgins has led the tributes to Sr Stanislaus Kennedy, describing her as a "woman of immense courage and vision".
Sr Stan died this morning aged 86 following an illness.
President Higgins said Sr Stan was a "fearless advocate for human rights and equality".
"Telling the truth of inequality required a woman of immense courage and vision. She was that.
"I had the privilege of meeting with her on many occasions over the years. On each occasion, she demonstrated the generosity of spirit, integrity and immense strength of character that defined her life's work," he said.
President Higgins added that Sr Stan’s contribution to Irish society was "profound".
RTÉ Archives: A day in the life with Sr Stan in 1985
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"From her work as the first chair of the National Committee on Pilot Schemes to Combat Poverty in the 1970s to her pioneering work with homeless women in Dublin leading to the founding of Focus Ireland in the 1980s, she has provided immeasurable support to those facing poverty and homelessness."
Taoiseach praises Sr Stan's 'tireless advocacy'
Taoiseach Micheál Martin also paid tribute to Sr Stan, describing her as a "true Christian who dedicated her life to helping those on the margins".
Mr Martin said Sr Stan was a "passionate advocate for helping those in need".
He said: "In 1985, she founded Focus Point now known as Focus Ireland, to support those experiencing homeless.
"I met her many times over the years and always admired her tireless advocacy and her ability to inform policy. She had great energy. She was a true Christian who dedicated her life to helping those on the margins.
"My thoughts and prayers are with her family and friends at this time."
Tánaiste Simon Harris described Sr Stan as a "formidable force in public life" who made an "immeasurable impact on Irish society".
He said she "devoted her life to tirelessly advocating on behalf of the most vulnerable people in Irish society".
"Sr Stan was always guided in her work by her faith and her compassion for others," he added.
Sinn Féin Leader Mary Lou McDonald said she was "deeply saddened" by Sr Stan's passing, describing it as a "huge loss".
"Sr Stan led a truly selfless, inspiring life centred on giving help and hope to those in need."
"For decades, Sr Stan was a tireless and compassionate advocate for the homeless, those inflicted by the scourge of poverty, and the welfare of vulnerable immigrants. She was a true champion of the poor."
'Powerful voice for compassion'
The Religious Sisters of Charity said Sr Stan worked tirelessly to support the homeless, immigrants and those in disadvantaged communities throughout Ireland and beyond.
Sr Patricia Lenihan, the Sisters of Charity's superior general, said she was a "powerful voice for compassion, equality and systematic change throughout her life".
"While there is a deep sadness, we are confident that her legacy of a life dedicated to the service of others in need, will continue to inspire us and generations of activists and social innovators in Ireland," she added.
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Focus Ireland, which was founded by Sr Stan in 1985, said in a statement that the charity was heartbroken by her passing.
"We would like to express our thanks to all the medical staff who showed great care for Stan in recent times," the statement said.
"Sr Stan, as she was affectionately known, was a visionary, a tireless advocate, and a compassionate force for change whose legacy will forever shape the heart, soul, and values of Focus Ireland."
Sr Stan was a powerhouse - McAleese
Former president Mary McAleese described Sr Stan as the best of the best who left a phenomenal legacy behind her.
She said Sr Stan was a powerhouse who, through her meticulous planning, ensured that her programmes will continue to survive and thrive.
Speaking on Today With Philip Boucher-Hayes, Ms McAleese said: "You look at Focus Ireland, you look at the young social innovators, sanctuary, all the Immigrant Council.
"All the things that she set up have a longevity because she forward planned. She's left a phenomenal engine behind her."
Ms McAleese said Sr Stan never shied away from issues, no matter how difficult things became.
She was a person of hope, prayer and faith, she added.
Ms McAleese said Sr Stan was a champion at drawing a person's attention to what they needed in a busy life to make that life fulfilled.
Tiglin founder Senator Aubrey McCarthy said before setting up Focus Ireland in 1985, Sr Stan worked with women who were homeless in Dublin.
Speaking to RTÉ's Drivetime, Mr McCarthy said she tried to have dinner every day with eight of these women and listened to them to "see why they felt excluded".
Senator McCarthy said Sr Stan made it her business to make sure they felt they had dignity and a sense of pride.
Meanwhile, Auxiliary Bishop of the Archdiocese of Dublin Paul Dempsey said Sr Stan was very frustrated that Ireland as a wealthy nation could still have the problem of homelessness.
"She was well ahead of her time with the whole immigrant issue," he said.
"She set up the Immigrant Council of Ireland in around the early 2000s. In a sense she was very prophetic with that."
"In a sense she was the voice of the voiceless in our society," Bishop Dempsey added.
Sr Stan a giant in what she achieved - Archbishop Martin
Archbishop Diarmuid Martin said Sr Stan was a giant in what she achieved.
He told the same programme: "I knew a joyful, elderly nun, physically slowed down, intellectually, extraordinarily sharp.
"But I don't know what her physical heart, but what the condition was as she got older, but her emotional, her heart of caring, it was a dominant thing that you saw with her."
The former Archbishop of Dublin said Sr Stan never got lost in the statistics because her real care was for individuals in the poor.
He said: "She had a capacity of trust in the poor, of knowing what the poor can do and how they, through surviving, they show extraordinary initiative.
"And you could see how happy she was when any of her people that she worked with did well. It was like a mother with their own children."
Minister for Housing James Browne said Sr Stan was a pioneer in the development of supports for those experiencing homelessness.
He said: "Sr Stan sought, throughout her entire life, to ensure the voices of those most marginalised or forgotten, including those who are homeless or new to this country, were raised.
"She charged the State to do better and deliver more, and she was right.
"My thoughts and prayers are with Sr Stan's family and loved ones at this time."
A book of condolence for Sr Stan will be opened in Mansion House tomorrow.
Lord Mayor of Dublin Cllr Ray McAdam said: "It is very sad news to hear of her passing, and on behalf of the citizens of Dublin I send my sincere condolences to her family and many friends."
The public will be able to sign the book from 10am to 4pm tomorrow and Thursday.
