The Speaker of the US House of Representatives has hailed the Good Friday Agreement as a "beacon to the world".
Speaking during an address to the Dáil, Nancy Pelosi reiterated that nothing should happen in the Brexit discussions that imperils the Agreement.
"On that holy day the world saw the dawn of peace in Northern Ireland that few had dared to dream.
"Let me be clear if the Brexit deal undermines the Good Friday Accord, there will be no chance of a US-UK trade agreement," she said.
"I say that hopefully that we would not have to face that reality," Me Pelosi added.
"As you face the challenges posed by Brexit, know that the United States Congress, Democrats and Republicans in the house and in the Senate, stand with you."
The US Speaker was invited to speak in Leinster House by Ceann Comhairle Seán Ó Fearghail.
She has joined a distinguished list of internationally high-ranking politicians to speak in the Dáil, including US presidents.
She said "we must ensure that nothing happens in the Brexit discussions that imperils the Good Friday Accord, including but not limited to, the seamless border between the Irish Republic and Northern Ireland."
Nancy Pelosi tells the Oireachtas that "we must ensure that nothing happens in the Brexit discussions that imperils the Good Friday Accord, including, but not limited to, the seamless border between the Irish Republic and Northern Ireland" pic.twitter.com/OjhsI2YGQB— RTÉ News (@rtenews) April 17, 2019
Ms Pelosi also expressed her support for Ireland's bid for a seat on the UN Security Council.
In her Dáil speech, she paid tribute to the contribution of the Irish in the US, recalling that the Irish served in the Lincoln army to "perserve the Union and save our nation".
She also praised U2 frontman Bono, who was in the public gallery, for his music and advocacy work.
Ms Pelosi also held discussions with Fianna Fáil, Sinn Féin and other members of the Opposition.
She is part of a delegation of high-ranking US politicians on a two-day fact-finding mission to Ireland.
Following her address in the Dáil, Ms Pelosi attended a meeting with President Michael D Higgins at Áras an Uachtaráin, during which they discussed Irish-US bilateral relations and the implications of Brexit.
This evening, Ms Pelosi attended a reception at Dublin Castle which was hosted by the Taoiseach.
Leo Varadkar said that the relationship between Ireland and the US "goes back centuries".
Speaking at the reception, he said people in the US helped to bring peace to Ireland and helped the country take its place "among nations of the world".
"When courageous men and women walked the path of peace, they did so knowing the US was alongside them," he said.
"The Good Friday Agreement was made possible by Americans of good faith and they helped pave the way for prosperity and gave young people their own future."
Mr Varadkar added that Ireland will do everything it can to avoid a hard border on the island of Ireland.
The US delegation includes several members of the Friends of Ireland caucus, including Congressman Richie Neal.
Tomorrow the delegation, who are bound in their determination to avoid the return of a hard border on the island, will visit Northern Ireland.