Prime Minister Boris Johnson called on Northern Ireland's political leaders to show leadership as Stormont returned from its three-year hiatus.
He refused to be drawn on how much additional funding there would be for the restored Executive - needed for the pledges made in the deal to restore Stormont - but promised his government would give the institutions his full support.
Mr Johnson rejected the idea that his visit was simply a "photo opportunity or publicity stunt" and said he wanted to mark the historic return of devolution.
In their first joint statement since taking office at the weekend, First Minister Arlene Foster and deputy First Minister Michelle O'Neill said funding to make the commitment in the deal a reality must be "quick".
Arlene Foster described their meeting with Leo Varadkar and Boris Johnson as "constructive".
“We need significant and sustained investment, not just this year but over a number of years. This is crucial in ensuring transformation in areas such as health and also our road and water infrastructures," she said.
Michelle O'Neill added: "All Executive Ministers are committed to working together to tackle some very serious issues in our society and across public services. But, quite simply, we need the money to make it happen.
“We have done our bit and I look forward to the fulfilment of the commitments made by the two Governments to let us get to work.”
Sloppy if DUP & SF didn't nail down the govt on how much genuinely new funding there'll be for public services. Far more important to ordinary folk here than all these commissioners & quangos. Undermines public confidence already in New Decade New Approach.— Suzanne Breen (@SuzyJourno) January 13, 2020
Taoiseach Leo Varadkar and Irish Foreign Minister Simon Coveney were also at Stormont.
Mr Varadkar said north-south and east-west cooperation would deepen as a result of the return of power sharing.
Speculation has abounded around the exact financial figure committed to Northern Ireland as part of last week's New Decade, New Approach deal.
DUP agriculture minister Edwin Poots said he expected the UK Government to deliver at least £2billion to support the power-sharing deal. He also suggested the Northern Ireland taxpayer may have to pick up some of the tab.
While not giving an exact figure Mr Johnson said his government were making "huge commitments" financially to Northern Ireland.
At a press conference in Stormont's Great Hall, the Prime Minister expressed his gratitude to the work of Stormont's parties and said they had compromised to facilitate the resumption of government.
He also commended Secretary of State Julian Smith for his role in the the negotiations.
"Now is the chance for the Executive and Assembly to deliver for people of Northern Ireland and deliver on the priorities of the people and that is above all improving people's healthcare, fantastic schools and making sure the streets are safe," Mr Johnson said.
"Of course we are going to be supportive but it's not just about money it's about leadership and what's so great about today is that Northern Ireland politicians put aside differences and stepped up to plate and showed leadership, that is a fine thing and the right thing."
The Prime Minister said the deal allowed politicians to develop on "a very, very promising set of circumstances for Northern Ireland".
"I want to make it absolutely clear that we in the UK Government will now work with this revived government to ensure we deliver on that potential through better infrastructure, better education and technology," Mr Johnson told the press conference.
"Using those three things to bring our whole UK together so all four nations benefit from the prosperity and growth we intend to deliver."
The Prime Minister, fresh off his crushing General Election victory, said the return of Stormont was a "great moment". He said that there was a period before Christmas where it looked like Stormont may not return.
"Never mind the hand of history on my shoulder," he said echoing Tony Blair's comments from 1998 when it appeared talks could collapse, "I see the hand of the future beckoning us all forward.
"I hope with goodwill and compromise and hardwork on all sides it will be a very bright future indeed."
Addressing the RHI Scandal, the Prime Minister said it was vital that public spending in Northern Ireland be "properly invigilated" and that there could be no repeat of the scandal.
He also moved to address fears around checks on goods coming from Great Britain to Northern Ireland after Brexit.
Mr Johnson said the only circumstances he could foresee checks on goods was if the goods were travelling on into the Republic and "we have not secured a zero tariff, zero quota trade deal with our partners in the EU."
He said the special arrangements for Northern Ireland would end after four years unless the Assembly wished to continue them.
The Prime Minister also promised to live up to his manifesto promise of "no further unfair prosecutions for people who served their country when there is no new evidence".
He said the legacy mechanisms contained within the Stormont House Agreement would provide balance and reassure both veterans and people "in search of truth".
He was also asked about the future of Julian Smith and the Northern Ireland Office. There has been speculation the Secretary of State could be moved off the brief in an expected Cabinet reshuffle in February. And there has long been suggestions the Scotland, Wales and NI offices could be rolled into one.
The Prime Minister would only say Mr Smith's future was "bright".
Mr Varadkar said one of the strengths of the deal was that there was an agreed approach on legacy issues. He acknowledged: "It is going to be difficult."
The Taoiseach said that he believed "the Good Friday Agreement is up and running again" and discussed with Mr Johnson co-operation between Ireland and Britain over the next couple of years and said a new post-Brexit trade deal should be in place as soon as possible.
"We are going to beef up and deepen co-operation between Britain and Ireland in the interests of everyone who lives on these islands," Mr Varadkar said.
Mr Johnson arrived a Stormont Castle flanked by Secretary of State Julian Smith and was greeted by DUP First Minister Arlene Foster and Sinn Fein deputy First Minister Michelle O'Neill.
As they posed for photographs Mr Johnson shook hands with Ms O'Neill and Mr Smith shook hands with Ms Foster.
Later he and Mr Smith attended a round-table meeting of the newly formed Executive.