Prime Minister Theresa May has backed Ireland's bid to host the World Cup in 2023.
Taoiseach Leo Varadkar revealed that Mrs May has written to the Rugby World Cup organisation, affirming the UK's support for the tournament to be hosted on the island.
Mr Varadkar was the only state leader attending in person as Ireland, France and South Africa presented their bids to stage the tournament in London on Monday.
President Emmanuel Macron has distanced himself from France's bid, declining to provide a video message as previously promised, while the French paraded support from the sons of the late All Black wing Jonah Lomu in their pitch.
"The main reason I'm speaking to Theresa May is around Brexit and also restoring the institutions in the north," the Taoiseach said.
"However, she has written to World Rugby, supporting our bid, and assuring them that the United Kingdom Government is behind it too.
"And I'm very grateful for the fact that she's done that.
"I'm really delighted to be here in person. I wouldn't have missed today for anything."
France made the bold claim that their bid would generate the most revenue for organisers Rugby World Cup.
But Mr Varadkar said: "Of course they are going to say that, but we would say otherwise.
"Our bid is fully backed by the Government. I'm here in person, the only head of government to be here in person."
Former Ireland captain Brian O'Driscoll hailed the Taoiseach's personal appearance, while musician and political activist Sir Bob Geldof and U2 singer Bono provided messages of support in a stirring video presentation.
"It's massively important and it shows the seriousness of our pitch," Mr O'Driscoll said of the Taoiseach's personal interest.
Ireland were able to allay organisers' concerns around the impact of Brexit, with a great deal of uncertainty still surrounding the land border between Northern Ireland and the Republic.
Mr Varadkar threw his weight behind the pledge to underwrite the entire cost of staging the 2023 tournament.
"I pointed out while there is plenty of uncertainty, one thing that is certain is that the common travel area will remain in place," the Taoiseach said.
"Our bid makes sense not just for rugby but also for the Irish taxpayer and the Irish economy.
"We've agreed that we'll underwrite the tournament costs.
"The French have said this would cost their taxpayers nothing, but we take the view that this is a good investment in our economy."
France's minister for sport Laura Flessel insisted other commitments prevented Mr Macron from attending.
"There was no video (from Macron), I'm here to represent the government," Ms Flessel said.
Jonah Lomu's sons Dhyreille and Brayley Lomu boosted France's bid, with seven-year-old Dhyreille born in Marseille when his father played for the club.
"That's where they saw their daddy playing rugby," said ex-France flanker Sebastien Chabal.
"They would love to come back in 2023 to experience the World Cup in the country where their father was so happy."