Dutch contractor BAM is to work with Irish civil engineer Wills Brothers on the $241m N5 highway between Westport and Turlough Road in County Mayo.
Work on the project includes construction of a 20km of dual carriageway from Westport to Turlough, and a 2.5km single carriageway link to the Mulranny.
The project aims to improve connectivity by diverting traffic away from the town centres of Castlebar and Westport.
BAM and Wills Brothers worked with Mayo County Council, Transport Infrastructure Ireland and Roughan & O’Donovan on the contract.
Peter Hynes, Mayo council’s chief executive, said: "This is the biggest single infrastructural investment ever undertaken in Mayo and will completely transform the landscape of this County.
"When completed the road will link Castlebar and Westport into a single cluster which will be on a par with growth centers identified in the current National Planning Framework. This Cluster will encourage investment into the County and bring Economic Development benefits for all of Mayo."
Preparatory site works are due to begin this year, and the project is due to completed by 2022.
Image: The signing of the deal