Transport Minister Leo Varadkar has turned the sod for the new €26m Longford by-pass.
The new road will be 2.6km long with five bridges.
The journey from Westport to Dublin will be reduced by approximately 15 minutes for 5,000 motorists who regularly use the route.
50 jobs will be created in the construction stage and the National Roads Authority said the bypass would result in the calming of traffic in one of the worst remaining bottlenecks in the country.
