The Minister for Transport, Martin Cullen, has turned the first sod for the M3 motorway just outside Navan, Co Meath.
Organisers kept the event low key to avoid opponents to the project turning up.
It is expected construction work on the motorway, which is estimated will cost between €800m and €1bn, will begin in a matter of days.
Described as the largest project of its kind ever undertaken in the country, the scheme will see 49km of motorway being built from Clonee in Co Meath to just north of Kells in addition to several access roads.
It is expected the motorway will be open by July 2010.
The motorway will bypass the towns of Dunboyne, Dunshaughlin, Navan and Kells.
However, it remains hugely controversial as it runs through the Tara/Skryne valley and objectors say it will damage the archaeological landscape of the Hill of Tara.
At the ceremony Minister Cullen denied the project is a gamble, as a case is being taken to the Supreme Court by An Taisce which could halt the scheme.
The minister said the people of the area had waited long enough for the motorway.
Today's move has been criticised by those opposed to the project.