The former Democrat contender for President was visiting Trinity College in Dublin, where she was picking up an honorary degree.
Speaking to students, Mrs Clinton, whose husband Bill was a former US President, spoke of the 1998 Good Friday agreement with mostly ended the bloodshed in Northern Ireland.
The pair were the first presidential couple to visit the country in 1995 before the Good Friday Agreement was signed, which helped to end three decades of violence in the country and the rest of the UK.
She said the Good Friday Agreement set an example for the rest of the world and was proof if what was possible when citizens came together to demand peace.
She said: “As the Brexit debate rages on, I continue to believe in the value of the European Union, and of a Europe that is whole, free and at peace.
“No matter the outcome of these discussions, Brexit should not be allowed to undermine the peace that people voted, fought and even died for.”
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The future of the Irish border is one of the most contested and debated issues facing negotiators in Brussels, with little insight into the conclusion at present
During her speech, she also cited the recent referendum over abortion laws in Northern Ireland, hailing the result as an “inspiring insight”.
A provision in the Irish constitution gave equal value to the life of an unborn foetus to that of its mother, with one permitting abortion in a limited number of circumstances.
Speaking of the referendum result, Mrs Clinton said: “It was an example of grassroots activism fueled by young people, and a triumph of the democratic process.”
However, a protest took place outside the building…