Motorists looking to bypass Limerick City could shave about 20 minutes off their journey at peak times after a new tunnel under the River Shannon opened.
The 605 million euro project is the length of five football pitches, with cars tolled 1.80 euro each way. It is estimated that up to 20,000 cars a day will be taken off Limerick's streets, although the tunnel has the capacity for 40,000.
Opening the tunnel, Taoiseach Brian Cowen said it was a historic crossing and described it as a key element in the state's national road infrastructure.
He said: "More than 1.4 billion euro will be spent this year on our national, regional and local road infrastructure. This level of investment provides a much-needed fiscal stimulus around the country and also helps maintain thousands of jobs."
The 675-metre (0.4-mile) tunnel took four years to complete and finished ahead of schedule. It forms the second phase of the Limerick southern ring road and connects Phase 1 at Rossbrien on the south side of the city to the N18 Ennis Road on the west.
Transport Minister Noel Dempsey said the tunnel was the outcome of many years of vision and effort.
He said: "It is one of the largest infrastructural projects ever undertaken in the mid-west region. It will provide a fourth crossing of the River Shannon in Limerick and improve access times for commuters to the city, as well as access to Shannon Airport, Galway, Cork, Kerry and Dublin."
National Roads Authority chairman Peter Malone said drivers would be able to drive under the Shannon to and from the airport, Ennis, and on to Co Galway.
He added: "Additionally, this project improves driver safety, enhances the quality of life for the residents of Limerick city while improving regional competitiveness. It will contribute positively to growth in tourism, high-end manufacturing and growing business services sector throughout the entire mid-west."