NI Deputy First Minister Martin McGuinness, Junior Minister Robin Newton and Irish Premier Brian Cowen led a VIP party of ministers at the ceremony.
The 12 km A1 Beech Hill to Cloghogue dual carriageway scheme involved substantial rock cuttings with 1.2 million tonnes of rock excavated to make way for the road.
Highlighting current 'good construction practice' all of this material was actually processed on site and then used in the construction of the new road.
Over two million man-hours were successfully worked on the project, which employed 500 people at the peak of the scheme.
The huge engineering scheme includes five flyover-type junctions and 17 new bridges and a 190m long viaduct carrying the new dual carriageway over the Newry River, the canal and the Tandragee Road.
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Stormont Transport Minister Conor Murphy, who performed the official opening, said: "This project is the final link on the key strategic route between Belfast and Dublin on Ireland's eastern seaboard and makes a substantial positive contribution to the social and economic well-being of our communities both north and south.
"The completion of this 12km stretch of new road from Beech Hill to Cloghogue, at a cost of just over £150million, marks the achievement of the Department's commitment to upgrade the remaining section of this road to full dual carriageway standard," he said.
Minister Murphy thanked Roads Service, Amey Lagan Roads, Lagan Construction and Ferrovial Agroman Ltd for their work completing the scheme ahead of schedule and also the local community for their assistance during the development and construction of the scheme.
Deputy First Minister, Martin McGuinness said: "Investments in road improvements, such as this key project, meet the Executive's strategic objectives in terms of promoting economic growth and competitiveness and balanced regional development.
"Transportation infrastructure is a key factor in sustainable economic growth.
"This stretch of road is an essential element of the development of dual carriageways and motorways connecting Larne through Belfast to Dublin and on to Rosslare," he commented.
"This will not only reduce transport times and costs, but create opportunities along the seaboard, linking the major ports and our economies.
"As we all face the outworking of the recession now is the time to build the economy, to take advantage of the opportunities and to deliver jobs and investment," the Executive Minister said.
This project was part funded by the European Union, TEN-T (Trans-European Network Transport) Programme.
(GK/BMcC)
