Dublin (AFP) - US President Joe Biden on Thursday declared "I'm home" as he was feted on a visit to his ancestral homeland Ireland, whose story he said proved the triumph of hope over despair.

Gearing up for a potential election rematch against Donald Trump, the Irish-American Democrat dwelt on the success of Irish emigres in carving out a new life far from home. 

The United States and Ireland were joined in "not just the hope but the conviction that better days lie ahead", he said in a speech to the Irish parliament, after his motorcade was cheered by large crowds in Dublin.

Seeming to draw a firm contrast to some US Republicans' dystopian vision for the future of America, Biden added: "We have the power to build a better future."

But, following a testy visit to Belfast prior to Dublin, Biden also issued a pointed warning that the UK "should be working closer with Ireland" to protect a 25-year-old peace deal in Northern Ireland.

"Political violence must never again be allowed to take hold in this island," he said to warm applause from the Irish audience, which included veteran nationalist leader Gerry Adams.

Adams, still a hate figure for many pro-UK unionists in Northern Ireland, hugged Biden after the speech, likely adding fuel to the unionists' fire after their former leader Arlene Foster said that Biden "hates the United Kingdom".

The White House pushed back strongly at that, and Irish Taoiseach (prime minister) Leo Varadkar said after meeting Biden in Dublin that the president was "keen to be supportive in any way that he can" to uphold the peace in Northern Ireland.

Biden backed efforts by the UK and Irish governments to end a unionist boycott of the devolved Belfast legislature, "but doesn't want to be overbearing or interfering either", Varadkar said.

- Gaffe -

Biden, who is only the second Catholic president in US history, earlier met Irish head of state Michael Higgins at his official residence.

Watched by fellow octogenarian Higgins, Biden tapped into his Irish roots for his message in the official guest book.

"As the Irish saying goes, your feet will bring you to where your heart is," he said he wrote, joking that he was going to stay in Ireland.

In parliament, he switched briefly from English to Irish, telling lawmakers and dignitaries: "I'm home."

Biden also took the opportunity to correct a gaffe that has gone viral, this time congratulating Ireland's rugby team on beating New Zealand's all-conquering All Blacks last year.

The night before, he said the Irish team had beaten the "Black and Tans", a notorious police force set up by the British to quell Irish independence fighters.

Biden was the fourth president to address the Oireachtas (Irish parliament) after John F. Kennedy, Ronald Reagan and Bill Clinton.

The parliament building, Leinster House, was built by Kennedy's aristocratic forebears, the Fitzgeralds.

Roots

Biden's own roots in Ireland were far more humble. Some of his forebears fled famine under British rule and congregated in hardscrabble Scranton, Pennsylvania.

The Speaker of the Dail lower house, Sean O'Fearghail, told Biden: "Today, you are amongst friends because you are one of us." 

Biden last met Varadkar in March in Washington for Ireland's St Patrick's Day celebrations, when the White House fountain ran emerald green.

Varadkar returned the favour with a banquet late Thursday in Biden's honour at Dublin Castle, the ancient seat of English and then British rule in Ireland.

Toasting Biden, the Irish premier reflected on the 25 years of peace in Northern Ireland since the Good Friday Agreement was signed in April 1998, after round-the-clock US mediation.

"Our mission now must be to keep this promise and honour the sacrifices that have been made by so many from across all communities in Northern Ireland and beyond," Varadkar said.

Before jetting home on Friday, Biden will head to Ballina in County Mayo, northwestern Ireland, another jumping-off point for ancestors who emigrated to Pennsylvania.

He still counts relatives living in the area, including Joe Blewitt, a third cousin, who works as a plumber.

"It's emotional, it's a very proud day for our family and for Ireland," Blewitt, 43, told AFP.

"Ballina's very special to him."