Under the development, a 2.3km section of the N59 at Barleyhill will be upgraded to Type 3 single carriageway standard. The project includes six roundabouts and two rail bridges over the railway line between Westport and Dublin.
A 20.3km of dual carriageway from Westport will be constructed to eastern Castlebar, along with a 2.5km single carriageway to connect to the N59 Westport to Mulranny national secondary road.
Development minister Michael Ring, who made the announcement, said that the investment - the largest ever in County Mayo - will benefit tourism in western Ireland.
The N5 is Mayo’s main route to the midlands and Dublin. Completion is expected in two years and the awarding of the construction contract is expected this autumn.
According to a report in the Western Mail newspaper in August, all four tenders were joint ventures. Submissions were received from SIAC/Colas, Wills/BAM, Roadbridge/SISK and Clonmel Enterprises/SACYR/John Craddock. Foxford-based Wills is the only local company in the running. Roughan O’Donovan-Faber Maunsell were appointed as consultant engineers.
Ireland's minister for Rural and Community Development has approved €241million for the N5 Westport to Turlough road project.