UK Prime Minister Rishi Sunak said the "real work starts now" as he told ministers in Northern Ireland to focus on day-to-day issues, not constitutional change, during a visit to the region on Monday.
The region’s government returned to work after a two-year impasse on Saturday, with Michelle O'Neill becoming its first leader from the Irish republican side.
Power-sharing was halted after Democratic Unionist Party (DUP) walked out in February 2022 in protest against post-Brexit trading arrangements, which it said undermined Northern Ireland’s place in the UK.
Under power-sharing rules, the administration in Belfast must include British unionists and Irish nationalists.
The party was coaxed into returning last week after the UK promised to eliminate most checks on goods moving to Northern Ireland from Britain.
Mr Sunak has insisted the priority now for leaders at Stormont should be delivering on the "day-to-day" concerns of people in Northern Ireland, not the possibility of a referendum on reunification with the republic of Ireland..
Speaking to broadcasters on a visit to Belfast, the Prime Minister dismissed any focus on a border poll after Ms O'Neill claimed one could be held in the next 10 years.
He said: "I had very constructive meetings this morning with the Executive, with political leaders across Stormont, and it is a historic and important day for the country, because Northern Ireland's politicians are back in charge, making decisions on behalf of their people, which is exactly how it should be.
"Now, our new deal gives them more funding and more powers than they have ever had, so they can deliver for families and businesses across Northern Ireland. And that's what everyone's priority is now.
"It is not constitutional change, it is delivering on the day-to-day things that matter to people."
Taoiseach Leo Varadkar, who was also at Stormont on Monday to mark the restoration of devolution, said Monday was about marking the return of power-sharing.
"Today is about marking the fact that the Good Friday Agreement, which we voted for in big numbers, north and south, is now functioning again," said Mr Varadkar, who has said he believes there will be a united Ireland in his lifetime.
"The Assembly is operating, the Executive, and hopefully the north-south [Implementation] bodies and ministerial council soon. I think today should be really about that rather than constitutional questions."
Visiting a school, Mr Sunak said the "future is undeniably brighter" for the children of Northern Ireland.
The DUP boycott left Northern Ireland’s 1.9 million people without a functioning administration to make key decisions as the cost of living soared and backlogs strained the creaking public health system.
Earlier, Northern Ireland Secretary Chris Heaton-Harris, who accompanied Mr Sunak on his visit, rejected claims by Stormont ministers that better funding is needed from the UK government as power-sharing returns.
The cabinet minister insisted the £3.3 billion ($4.14 billion) package offered by Westminster was "ample" for the Executive to "get on with the job".
But the Belfast administration says it is not enough.
Deputy First Minister Emma Little-Pengelly, of the DUP, said the ministers in Belfast would be “seeking to ensure the UK government provides sufficient funding in a package to fulfil its promises on public sector pay”.
'Very positive day'
The UK and the Republic of Ireland both act as guarantors of the landmark Good Friday peace agreement.
Mr Varadkar described it as a "very positive day".
The two leaders met for a bilateral meeting at Parliament Buildings and Mr Sunak travelled the short distance to Stormont Castle for a meeting with Northern Ireland's first nationalist First Minister, Sinn Fein's Michelle O'Neill, and her DUP counterpart, deputy First Minister Emma Little-Pengelly.
As the meeting got under way inside the castle, Mr Sunak told the joint heads of the ministerial executive: "It has taken a lot of hard work and indeed courage to get us sitting round this table.
"Today isn't the end; it's the beginning, and the real work starts now."
Sinn Fein leader Mary Lou McDonald said "change is all around and must be managed" following her meeting with Mr Sunak.
Ms McDonald posted a picture of herself and new First Minister Michelle O'Neill meeting Mr Sunak and Northern Ireland Secretary Chris Heaton-Harris.
She said: "Early meeting with @10DowningStreet @RishiSunak in advance of first Executive.
"Change is all around and must be managed.
"Shared commitment to partnership and respect at heart of progress."
The new administration is led by Ms O’Neill of Sinn Fein, the party allied with the Irish Republican Army during Northern Ireland’s decades of violence known as The Troubles.
Her appointment was historic, marking the first time an Irish nationalist, who aspires to take Northern Ireland out of the UK and unite it with the republic, has held the post.
In practice, nationalists and unionists will continue to govern in uneasy balance.
The post of deputy first minister – held by Ms Little-Pengelly of the DUP – is officially equal to the first minister, and neither can govern without the other.
