US President Joe Biden was winding down an emotive visit to Ireland with a visit Friday to the celebrated Roman Catholic pilgrimage site of Knock, before delving deeper into his Irish family history.

The Irish-American Democrat will later wrap up his three-day trip with an outdoors address to thousands of well-wishers at St Muredach's Cathedral in Ballina, northwest Ireland.

In 1828, Biden's forebear Edward Blewitt sold the bricks that went into the construction of the Catholic cathedral, using the money to fund his family's later emigration to the United States.

The picturesque riverside town of Ballina is proudly displaying US flags and red, white and blue bunting as locals buzz in anticipation of the visit.

Ballina commissioned a five-metre-high mural of Biden when he won the 2020 presidential vote - and much of the trip has appeared designed by the White House to build up to a 2024 re-election bid.

Biden 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."

The surrounding county of Mayo was the ancestral homeland of one branch of the Biden family, and the president was to tour a genealogy centre to find out more about his origins. 

Friday's first stop for America's second Catholic president was Knock Shrine in Mayo, a popular pilgrimage site since locals claimed to have seen an apparition of the Virgin Mary in 1879.

Pope Francis visited the shrine in 2018.

On Thursday, Biden declared in a speech to the Irish parliament: "I'm home."

Ahead of a potential election rematch against Donald Trump, the president 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.

Gerry Adams selfie

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 parliamentary audience, which included veteran nationalist leader Gerry Adams.

Adams hugged Biden after the speech and the pair posed for a selfie which the former Sinn Fein leader posted on Twitter.

Adams is still a hate figure for many pro-UK unionists in Northern Ireland for his alleged involvement in the paramilitary Irish Republican Army (IRA).

The imagery fuelled accusations - denied by the White House - that Biden is "anti-British".

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 backs 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.

South of the border, there has been no ambivalence in the adulatory welcome Biden has received at every stop.

Cheering crowds lined the streets of Dublin Thursday as Biden progressed to a meeting with Irish head of state Michael Higgins.

Watched by fellow octogenarian Higgins, Biden tapped into his 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 - and joked that he was going to stay in Ireland.