The new M3 officially opened to traffic this afternoon, two months ahead of schedule.
It has cost €1bn to build and the National Roads Authority says the new road, which has two tolls, could cut an hour or more off the peak journey times between Dublin and Cavan.
The 60km of motorway runs from Clonee on the Dublin/Meath border up to the Meath/Cavan border, bypassing the towns of Navan, Kells and Dunshaughlin.
The road was built as a public-private partnership and it will cost cars €1.30 at each of the two tolls, while large trucks will pay €3.40.
It is one of the biggest road projects in Europe to open this year.
However, the motorway has been one of the most controversial projects of recent years.
Campaigners staged protests on the route and in the courts to stop it going through the Tara Skryne Valley.
They described today as 'a sad one' for Ireland and its heritage.
However, Minister for Transport Noel Dempsey insisted the motorway will improve safety and the quality of life for those who will use the road and for those living in the towns now bypassed.