By Michelle Devane and Cate McCurry, Press Association
US House of Representatives speaker Nancy Pelosi has said that Brexit must not result in the return of a hard border.
The senior Democrat made the remarks in a speech to members of the Irish parliament on Wednesday afternoon as part of the Dail's centenary commemorations.
TDs, senators and former politicians - as well as U2 singer Bono - were in attendance to hear Ms Pelosi's address.
Ms Pelosi said that Brexit must not undermine or challenge the Good Friday Agreement or allow the return of a hard border.
She added that politicians in the US stand with Ireland as it faces challenges posed by Brexit.
"We must ensure nothing happens in the Brexit discussion that threatens the Good Friday accord," she said.
"If the Brexit deal undermines the accord there will be no chance of a US-UK trade agreement."
Ms Pelosi is part of a delegation of high-ranking US politicians on a two-day fact-finding mission to the country.
The congressional group includes several members of the Friends of Ireland caucus, including congressman Richie Neal.
Ms Pelosi was invited to speak in Leinster House by Ceann Comhairle (speaker) Sean O Fearghail.
Ms Pelosi said two decades of peace cannot be jeopardised by the Brexit process.
"We treasure the Good Friday accord because it is not just a treaty, it is an ethic, it is a value, it is an article of faith for us, it is a beacon to the world," she said.
"We treasure the Good Friday accord because of what it says is possible for the entire world and the reason to hope that in every place the dreams of reconciliation is possible for them too.
"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."
She added that children born 21 years ago know only peace.
"We cannot jeopardise that," she added.
Mr O Fearghail thanked Ms Pelosi and the US delegation for the visit, particularly as the UK prepares to leave the EU.
"I am delighted this senior delegation has taken time to visit to see first-hand the implications for us in a difficult Brexit.
"I wish to thank you and all our friends in the House of Representatives."
He added: "Our friends in the US Congress have been steadfast over the last 100 years, especially during the Northern Ireland peace process.
"That help was crucial in the success of the peace process two decades ago and continues to be vitally important today.
"That support will be remembered with deepest gratitude."
On Tuesday, Ms Pelosi met Taoiseach Leo Varadkar and the country's foreign affairs minister Simon Coveney on the first day of her visit.
Ms Pelosi is the third most powerful politician in the US.
The US politicians will also meet President Michael D Higgins at Aras an Uachtarain and attend a dinner, hosted by Mr Varadkar, at Dublin Castle, on the final day of their trip to the Republic.
The delegation will visit Northern Ireland on Thursday.