The Newlands cross flyover has been officially opened by Transport Minister Paschal Donohoe - meaning motorists can now drive from Belfast to Cork without encountering a single traffic light.
The upgrade is part of a €282m N7 Newlands Cross/ N11 Arklow to Rathnew Public Private Parnership (PPP) scheme.
The junction is one of the busiest in Ireland with around 80,000 vehicles passing through each day and is the first PPP scheme to be signed since 2007.
The road will open to traffic on Thursday.
This afternoon 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 road was build under a Public Private Partnership with the State and BAM Group Ireland.
Speaking at the official opening, Theo Cullinane, BAM Group Ireland Chief Executive said: “Motorists can now travel the length of the country without traffic lights, a development which is truly remarkable and unique across Europe. I would like to pay tribute to commuters, local residents and business owners for their co-operation and patience while construction was underway, and I can assure them that the new junction will have a positive impact on the area."
Over 950 construction workers were involved in the project through its duration.
National Roads Authority CEO Fred Barry thanked the public for their on going patience during construction.
The flyover is 670m long with three lanes on each side. The NRA say the road has been "future proofed" to allow for a Luas to pass under.
Work on local access wil continue for around another three months.