Sir Keir Starmer says it is time to reset the relationship between the UK and the Republic of Ireland.
On Saturday, he became the first British prime minister to visit the Republic of Ireland in five years.
Downing Street said the visit marked a “new era of cooperation and friendship” between the UK and Ireland.
The prime minister held talks with Taoiseach (Prime Minister of Ireland) Simon Harris who was elected in April, just three months before his British counterpart – both new to the role.
Ukraine is on the agenda
A full day’s engagement in the Irish capital ended with an attendance at the Republic of Ireland v England football match at the Aviva Stadium in Dublin.
Sir Keir said pressing international issues including Ukraine and the conflict in the Middle East were also on the agenda of the meeting.
“In the face of Russian aggression, we must stand together, to coordinate, stand with our allies and give Ukraine all the necessary support for as long as necessary,” he said.
“Regarding the Middle East, we need a ceasefire so that the remaining hostages can get out, so that much-needed aid can go into Gaza, and we can go down the road to a two-state settlement, which in my view is the only lasting settlement that will bring eternal peace.”
Sir Keir Starmer arrived at Farmleigh House at Phoenix Park just before 1pm local time.
He met Mr Harris who wished Sir Keir a “hundred thousand welcome” as the two leaders shook hands.
The Taoiseach, who described Sir Keir as his friend, said: “It is a great honor to host you here today on your first official visit to Ireland as prime minister.”
He added: “We’ve both said we want to take the UK-Irish relationship on a new path and I’m very happy with the time that’s been given to us since I’ve been in office, and I’ve tried to respond well.”
Harris said the reset in relations should be placed in “peace and prosperity, mutual respect and friendship”.
He said his “most important duty” as the leader of the British and Irish governments is to be the guarantor of the peace process in Northern Ireland.
Sir Keir said a reset in relations between the UK and Ireland “could be meaningful, could be profound”.
“Obviously it has to cover the Good Friday Agreement and I am very serious about our joint role,” he said.
‘Can we get Declan Rice back?’
At Dublin’s Farmleigh House, Sir Keir signed the visitors’ book.
They moved to the walled garden of the property, where they walked together in conversation.
The youngster, who is receiving medical treatment in England, presented the two leaders with Irish and England football scarves to change into ahead of the match at the Aviva later.
He also changed his football shirt.
Sir Keir held up an Ireland shirt with his last name on it and laughed when asked if he would wear it to a later match.
He said: “It will be seen on my nine sides. It will be worn proudly in north London before too long.”
Looking at a British shirt with “Harris” on the back, the Taoiseach joked: “I never thought I’d see my name.”
Meanwhile, Sir Keir said “absolutely not” to shouts of “can we have Declan Rice back?”
Speaking about the England footballer, he said: “No, absolutely not. He’s fantastic.”
‘Our relationship never reached its full potential’
Sir Keir and Mr Harris met at Checkers in mid-July after Labour’s general election victory.
They discussed the division through the UK Heritage Act, and were photographed having a pint of Guinness.
The Taoiseach is the first international leader held by Sir Keir after coming to power, a fact that demonstrates the prime minister’s personal commitment to strong UK-Irish relations.
Saturday’s visit follows the prime minister’s visits to Berlin and Paris last week to continue the UK’s reset with key European partners.
Sir Keir and the taoiseach will also meet Irish business leaders to encourage better bilateral trade and investment to drive growth.
The last British prime minister’s visit to the Republic of Ireland was by Boris Johnson in September 2019.
Today, under the former Conservative government, relations between the UK and Ireland are said to be at their lowest ebb for decades due to differences over Brexit.
Ireland also launched an international legal case against the UK over the Heritage Act which Sir Keir has now promised to repeal.
Analysis: Gareth Gordon, BBC News NI political reporter
This visit, with its sporting conclusion, took political football to a new level.
And as a cliché it really is a game of two halves.
In the predecessors of Leo Varadkar and Boris Johnson’s relationship between the two countries reminded some of the famous line of comments “they thought it was over – now it’s over”.
Then, Sir Keir Starmer walked onto the pitch and it seemed like a whole new ball game.
On the two major issues facing the two countries today, a Labor government coming to power could offer a fresh start.
Of course mood music is better.
Back in 1995, the English football team played in Dublin, but the “friendly” match had to be abandoned after fans rioted.
Different results have been expected for 29 years, whichever side wins football.