The Newlands Cross flyover on the N7 opened to traffic today.
The new road layout means that motorists can now travel from the outskirts of Cork to Belfast without encountering a single traffic light.
The project in west Dublin was completed four months ahead of schedule and within budget.
An estimated 85,000 vehicles pass through Newlands Cross every day, making it one of the busiest junctions in the country.
The flyover was officially opened by Minister for Transport Paschal Donohoe and South Dublin mayor Fintan Warfield on Tuesday.