The mother of two died surrounded by her family at Milford Hospice in Limerick in the early hours of this morning.
Ms Phelan’s legal case against the HSE and a US laboratory exposed the CervicalCheck scandal in which hundreds of Irish women were given incorrect negative smear test results, with some of them only finding out much later that their results were positive.
It ultimately prompted a series of reviews of the cervical cancer screening programme CervicalCheck.
Paying tribute, Taoiseach Micheál Martin said it was “very sad news” and extended his sympathies to Ms Phelan’s family.
Speaking to Claire Byrne on RTÉ this morning, he especially praised Ms Phelan for refusing to sign a non-disclosure agreement which would have meant to her case did not go public.
"I think this is very, very sad news,” Mr Martin said. “I think she was a woman of extraordinary courage and integrity for the people of Ireland, but not just the woman of Ireland but women globally.
"To her husband and family, we extend our sympathies.
"I think in the history of this country, I think her actions particularly in terms of not signing a confidentiality agreement at that particular time, in her statement on the steps of the High Court, live long in the memory as an example of someone who stood up against the system and the normal conventions of actions and so on. To say ‘no, I'm not signing that, I want this revealed in the public interest’. And she stood up for the public interest."
Ms Phelan case prompted other women to come forward, and raised questions about the quality of the programme, about how women should be involved and informed about their own healthcare, and the issue of open disclosure.
Despite receiving a terminal cervical cancer diagnosis, Ms Phelan has been actively campaigning for better healthcare and better accountability when mistakes are made in Ireland's healthcare system.
She also supported the passage of the Dying with Dignity Bill, which aims to legislate for assisted dying in Ireland, through the Irish parliament.
Ms Phelan was awarded the freedom of Limerick earlier this year, and was the subject of the documentary Vicky, which was released last month. She was named as one of the BBC's 100 most inspiring and influential women around the world in 2018.
More to follow...