US President Joe Biden, winding down an emotive visit to Ireland, broke down in tears Friday during a chance encounter with the priest who read the last rites to his late son.

The Irish-American president was on a visit to the celebrated Roman Catholic pilgrimage shrine at Knock, northwestern Ireland, on the final day of a three-day tour of his ancestral homeland.

Biden will later wrap up his trip with an outdoors address to thousands of well-wishers at St Muredach's Cathedral in Ballina.

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.

At the Knock Shrine, organizers made the last-minute discovery of a link between the Biden family and one of their priests, Father Frank O'Grady, who returned to Ireland after years serving as a chaplain in the US army.

The president's son Beau Biden died of brain cancer aged 46 in 2015. Father O'Grady administered the Catholic rites at the Walter Reed Army Medical Center outside Washington.

O'Grady was not on the official guest list but was given hurried security clearance.

Biden "wanted to meet him straight away, he dispatched a Secret Service agent to go and find him," Knock Shrine parish priest Father Richard Gibbons told the BBC.

"He got the shock of his life, to come over, so that was a wonderful spontaneous thing that happened.

"He (Biden) was crying, it really affected him and then we said a prayer, said a decade of the rosary for his family.

"He lit a candle and then he took a moment or two of private (reflection) for prayer."

O'Grady told Irish broadcaster RTE that he had a "nice chat" with Biden, the president's remaining son Hunter and sister Valerie Biden Owens for about 10 minutes in the Knock Shrine chapel.

"He was delighted to see me and I was delighted to see him. He gave me a big hug. Hunter gave me a big hug. It was like a reunion," the chaplain said.

"He told me he appreciated everything that was done for him (Beau Biden)."

The US president went on to visit the Mayo Roscommon Hospice nearby with his son, sister, and Irish cousin Laurita Blewitt.

In 2017, he came for the building's groundbreaking, and a plaque there commemorates Beau Biden.

'I'm home'

Biden was to head on to the picturesque riverside town of Ballina, which was proudly displaying US flags and red, white and blue bunting as locals thronged the streets in excited anticipation.

Ballina commissioned a five-meter-high (16-foot-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.

Blewitt descendants still live in the town, where the Mocha Beans cafe changed its shop sign to read "Mocha Biden" for the occasion.

"That buzz is incredible around Ballina today," the cafe's owner Trevor Mangan told AFP.

As a baby, Flori Garvin was given a cuddly toy donkey by Biden when the Democrat visited Ballina as vice president. Now aged seven, she was back with her grandmother, Elizabeth Robinson, 63.

"She hasn't stopped talking about it," Robinson said. "She thinks she's going to see him herself."

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

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 are 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.

The audience included veteran nationalist leader Gerry Adams, who hugged Biden after the speech. 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 fueled accusations that Biden is "anti-British".

Irish Taoiseach (prime minister) Leo Varadkar echoed the White House in denying that. He insisted that Biden was keen to help protect the peace process without being "overbearing or interfering".

© Agence France-Presse