Work has started on the €250m upgrade of the Newlands Cross junction in Dublin and the N11 in Wicklow.
It is being financed by the first public private partnership in six years.
The Newlands Cross junction upgrade on the N7 will mean the removal of the last set of traffic lights between Belfast and Cork.
The junction is to be converted into a flyover, which is intended to remove a bottleneck in the national roads system affecting 80,000 cars a day.
It will provide continuous free-flowing traffic between the M50 at the Red Cow and the cities of Limerick, Cork, Kilkenny and Waterford.
The National Roads Authority says that towards the end of the summer there could be more congestion due to roadworks, but has promised to keep three lanes open in peak periods.
The N11 works will involve adding 16km of dual carriageway to the N11 between Arklow and Rathnew.
Minister for Transport Leo Varadkar said the upgrades are part of a series of projects announced recently that show the tide is turning for the availability of public investment.
He said both projects will be completed within two years.