The Irish government has announced substantial new funding worth €800m (£685m) for cross-border projects.
The cabinet in Dublin approved the plans, which include €600m for the upgrade of the A5.
It's also allocating €50 million for the redevelopment of Casement Park in west Belfast.
Taoiseach (Irish Prime Minister) Leo Varadkar said: “The commitments we’ve made today are about potential for cross border communities.
"It’s about understanding that whatever constitutional future of Ireland and Northern Ireland as a whole may be, investing in people and infrastructure, quality of life, in opportunity for generations to come are all of our shared and a common good that we can all achieve by working together."
On the A5 upgrade, Mr Vardkar said an allocation of €600m would "bring connectivity between north and south in line with the rest of the island".
Mr Varadkar along with his partners in government, Tánaiste (Deputy Prime Minister) Micheál Martin and Green Party leader Eamon Ryan addressed the media in Dublin.
The funding announcement also includes updates on existing projects such as the Narrow Water Bridge.
Tanáiste Micheál Martin described it as "a special and important moment".
He said: "Iconic and strategically important projects that have been talked about for decades have either been given the go-ahead to start building or been given significant funding commitments to allow them to move forward with the help of partners, as in the case of the A5 upgrade."
Mr Martin added that he was "particularly pleased" to give the green light to the Narrow Water Bridge project.
He said the money for Casement Park was a "major investment in the social fabric of a part of Belfast that has suffered from disadvantage over many years".
The A5 is the main route from Derry to Dublin
Irish Transport Minister Eamon Ryan said the funding was "timely and important" with the return of the Northern Ireland Executive and North-South Ministerial Council begins to meet.
Mr Ryan said the A5 upgrade and hourly train services between Belfast and Dublin would "improve and strengthen" connectivity.
Mr Ryan said the funding was just "scratching the surface" and would deepen the connections on the island of Ireland.
Sinn Féin's Paul Maskey has welcomed the funding allocation for Casement Park.
The West Belfast MP described it as another "positive step" towards the building of a "first class, state of the art" sporting facility.
The SDLP has said the Irish government's financial commitments would "help surcharge some projects that have been talked about for years".
The party's leader Colm Eastwood called for the Stormont executive to publish its commitments to key projects.
DUP leader Sir Jeffrey Donaldson has welcomed the financial support from the Irish government but said the UK should be making commitments.
He added: "It is not the job or the responsibility of the Republic’s government to provide financial support for the provision of public services and general Northern Ireland infrastructure. That is a matter for the UK."
Speaking before the A5 funding was confirmed Niall McKenna from the campaign group Enough is Enough which was set up after the death of John Rafferty in 2020 said he was "confident" the new road upgrade would happen.
He was speaking on BBC's Good Morning Ulster programme.
"We are extremely confident that it's going to happen this time - we can see everything moving in the right direction and we are very confident it's going to happen this time," he said.
"So many have been affected by death on this road [A5] - this road is quite literally killing our members and we have a duty of care to see that end."
Mr McKenna said there had been a "sea change" in attitude.
He added: "The general public are not prepared to accept the non-delivery of it [A5] anymore."
Hamilton Hazzard, a farmer in Omagh for over 30 years, is opposed to the A5 road plan.
“I’m probably going to lose 13% of my acreage off my farm – that’s not the whole reason why I object to it – the existing road should have been improved years ago and we’ve constantly advocated for it’s improvement.
"We are certainly against the proposed scheme, we’re not against improvement of the road in fact we’re for the improvement of the road,” he added.
Mr Hazzard said road improvements could be made without the loss of land.
He added: “The road should have been improved with plenty of passing options, straighten it where it’s appropriate and lay-bys placed and you could have had an excellent road without the need to sacrifice 3,000 acres of prime agricultural quality land.”
The A5 road upgrade project is to get the full funding once promised
The announcements today are part of the Shared Island Project founded by Mr Martin when he was taoiseach in the coalition government in 2020.
The Programme for Government and revised National Development Plan (2021-2030) announced by the Irish government both include substantial all-island investment.
The total all-island investment commitment is more than €3.5bn (£3bn) by 2030 with new funding worth €800m for cross-border projects announced on Tuesday. The funding is through the government’s Shared Island Fund; Project Ireland 2040 funds; resourcing for North/South cooperation; and the PEACEPLUS programme, delivered with the European Union, UK Government and Northern Ireland Executive.