Nancy Pelosi, the speaker of the US House of Representatives, has said two decades of peace in Ireland cannot be jeopardised by Brexit.
In a historic address at Leinster House, she told current and former Oireachtas members that the Good Friday Agreement is a "beacon to the world".
"We treasure the Good Friday Accord," she said.
In a contribution that focused mainly on the strong ties between Ireland and the US, Ms Pelosi also gave her backing to the Government's bid to win a seat on the UN Security Council.
She urged Irish politicians to be leaders on climate change and praised Ireland as a modern nation that has "blossomed", citing the recent referendums on marriage equality and abortion.
The event to mark the 100th anniversary of the first sitting of Dail Eireann was also attended by other member of the US Congress who travelled as part of Ms Pelosi's delegation.
She said both Ireland and the US "know the joy of independence" and both "endured the traumatic experience of civil war and the satisfaction of rebuilding our nations".
"It is these mutual experiences that our nations affirm for each other and to the world our democratic values and commitment to freedom," she added.
"When Ireland proudly proclaimed its independence, our people stood together."
To applause, she repeated her view that there will be no trade deal between the US and UK after Brexit if there is any possibility of the Good Friday Agreement being undermined.
"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," she said.
Ms Pelosi also praised Ireland's efforts on green technology but insisted that both Ireland and the US could do more on climate change and could do so together.
She said they "must do better and must do more and can to it together" with the urgency the issue demands.
The speaker described climate change as a national security issue throughout the world.
She said it was a public issue in terms of clean air and water, an economic issue in terms of green jobs and reducing income inequality and a security decision "to keep us safe".
Welcoming Ms Pelosi and her delegation to Dublin, Ceann Comhairle Sean O Fearghail said it was significant that they came as the Dail celebrated its 100th anniversary.
"The past century has seen Ireland change and evolve into a country which would perhaps be unrecognisable to those who sat in the Mansion House in 1919," he said.
"We have taken our place among the free nations of the world, intrinsically linked to the European project and deeply proud of our role in UN peacekeeping.
"We are outward-looking and anxious to welcome those coming to our shores, temporarily or long-term.
"We have sought to create an inclusive, tolerant society, mindful of our rich heritage but respectful of difference."
Mr O Fearghail added: "We still have many challenges facing us, and we have some way to go yet to be the fair and equitable society we all strive for.
"We have travelled that path with international friends and allies."