The Newlands Cross flyover on the N7 has been officially opened by Minister for Transport Paschal Donohoe.
The road will be open to traffic on Thursday.
The upgrade will provide a continuous free-flowing route between the M50 Red Cow interchange and Limerick, Cork, Kilkenny and Waterford.
An estimated 85,000 vehicles pass through Newlands Cross every day, making it one of the busiest junctions in the country.
Minister Donohoe said: "The Newlands Cross junction upgrade is open before schedule and it's an exciting development for regular commuters, for motorists, for hauliers and for anyone who remembers the frustrating delays and frequent tailbacks that used to occur here at Newlands Cross.
"The new junction will now also connect our major cities, providing a continuous free flowing route between the M50 and the cities of Limerick, Cork, Kilkenny and Waterford.
"The Newland Cross project is part of the €282m N7/N11Arklow to Rathnew PPP Scheme.
"This project was the first PPP road project to be signed since 2007."
Mr Donohoe said the flyover was a gigantic project to deliver, adding credit is deserved to everyone involved in doing so.
The Minister also said "today marks a huge sign of progress in our economy."
The flyover is 700m long with three lanes on each sides.
The project was completed four months ahead of schedule and within budget.
Speaking on RTÉ’s Morning Ireland, Sean O'Neill, Head of Communications at the National Roads Authority, said the new road would make a huge difference to the lives of many people.
"It’s going to be a major relief come Thursday morning for the morning commute," he said.
"In the early hours of Thursday morning the contractor is going to do a major traffic shift, which is taking everyone from the temporary road alignment that they are experiencing today, and travelling up and over Newlands Cross for their morning commute on Thursday morning."