The 40-year-old, who took over from Martin McGuinness in January this year, said the decision by Britain to leave the European Union (EU) had “changed everything” during the annual oration at Glasnevin cemetery in Dublin on the 101st anniversary of the rebellion.
She said: "The Brexit referendum result has swept away many of the previous political assumptions about the constitutional, political and economic status quo in Ireland.
"Ireland's political landscape, North and South, will change dramatically – and this poses a severe threat to the Good Friday Agreement and the political and economic future of the island.
Michelle O'Neill (L) with Gerry Adams
"This has brought the issue of Irish reunification firmly back on to the political agenda.
"The people of the North clearly voted to see their future in the European Union in the referendum last June. Those who voted remain came from all walks of life. They were nationalists, unionists, republicans and others.
"They did so not because the EU is such a great institution, but because it is in their best interests politically, socially and economically and because they did not want to see any strengthening of the border in Ireland.
Martin McGuinness' life in pictures Tue, March 21, 2017
The former deputy first minister of Northern Ireland, has passed away aged 66
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Martin McGuinness and Tony Blair look on at Stormont Parliament Buildings in Belfast in 2007
"The British government's reckless Brexit agenda offers nothing to the people of the north who are being dragged out against our will.
"The Brexit referendum was driven by the narrow, right-wing interests of the most hard-line elements of the British Tory party and the far right of Ukip. These parties have never shown any regard for the North of Ireland or our people.
"Since the referendum, Theresa May and her government have done nothing to indicate this has changed. In fact, they have blatantly ignored the democratic will of the majority of the people of the north."
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Sinn Fein's leader in Northern Ireland Michelle O'Neill
Mrs O'Neill reiterated her call for Irish Taoiseach Enda Kenny to press for Northern Ireland to secure special designated EU status post-Brexit.
"What Enda Kenny must now do is stop standing by and start to take serious his moral duty and political responsibility to act in the national interest of all Irish citizens," she said.
She added: "We are entering a defining period in Irish political history. The opportunities for real change are within our grasp.
"The old certainties are gone. The grip of the old parties is loosening."
Michelle O'Neill, Sinn Fein's leader in Northern Ireland
The Mid Ulster Assembly member concluded: "We want to be in government in both parts of Ireland in order to serve the Irish people and to deliver a Real Republic.
"To finish the business of 1916 and the great challenge of the Proclamation in uniting all the people of this island, whatever their background or tradition, in equality and mutual respect together."
Almost a week ago Mrs O’Neill called for another Assembly election saying that there was little prospect of an agreement in the current round of talks aimed at saving devolution.
She said: "We remain committed to trying to deal with the issues which are there.
"We have set out what we need to see delivery on and we need to see a different approach from the DUP and, indeed, from the British Government.
"But post-Friday, I think it is over to the electorate to have their say about the future.
"But clearly Sinn Fein want to make these institutions work, but they have to work for all our citizens."