The journey time from Dublin and Belfast will be cut to only two hours with today's opening of a new dual carriageway between Dundalk and Newry.
The ribbon will be cut on the 14 kilometre-long A1/N1 dual carriageway by Minister for Foreign Affairs Dermot Ahern and Northern Ireland Regional Development Minister Conor Murphy.
The €154 million project will link the A1 at Cloghoge in Co Armagh to the N1 at Dundalk.
Some 4.6 kilometres of the scheme is north of the Border and the remaining 9.4 kilometres is in the Republic. The section of the scheme south of the Border has a speed limit of 120km/h.
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The Irish and British governments funded the project with assistance by the European Union from the Trans-European Transport Networks Budget.