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.