Construction is under way on the 13.5km bypass of Tralee in Co Kerry.
The Tralee bypass is one of only six national projects going ahead this year.
The €97m project includes an 8km dual carriageway around Tralee town and a 5.5km single carriageway link to the Killarney Road.
It will take 21 months to build and up to 150 people will be employed during construction.
Business and tourist interest groups have welcomed the development, which is seen as essential for the region's economic growth.
As well as cutting traffic levels in Tralee town by 25%, the new bypass will improve access to Kerry Airport as well as travel to and from Killarney.
It is expected that almost 19,000 vehicles per day will travel the route by 2026.
Minister for Transport Leo Varadkar turned the sod today to mark the official start of the project.
Mr Varadkar said: 'This project will dramatically improve safety benefits to motorists and vulnerable road users alike resulting in a predicted 14% reduction in accidents.
Other national projects going ahead this year include the €98m N5 road project in Cork, the N3 Belturbet Bypass, the N5 Longford Bypass, the N4 Downs Grade Separation project, and the N52 Carrickbridge to Dalystown project.