Taoiseach Brian Cowen performed the official opening shortly after 1pm and the tunnel will be open to traffic between 4pm and 5pm.

Mr Cowen described the tunnel under the Shannon as "an historic crossing" and said: "It is is a key element in our national road infrastructure and is vitally important to the people of the Mid-West.

"As the world economy improves, it is important to remember all the positive things happening in our country. More than €1.4 billion 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 delivery of these vital routes has transformed our national road network beyond all recognition. This investment will make us more competitive and help support jobs and economic growth as the global economy recovers," the Taoiseach added.

Transport Minister Noel Dempsey said the Limerick Tunnel is the outcome of many years of vision and effort.

"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.

"By the end of 2010," Minister Dempsey said, "all our major cities, together with towns and villages between them will be accessible via a world-class road network, which will offer safer and more reliable travel for people. I wish a safe journey to everyone travelling on this motorway and I ask all who drive on it to behave courteously to other road users."

Mayor of Limerick Cllr Maria Byrne said the opening of the tunnel was a "good news day" for Limerick and Minister of State Peter Power said the benefits of the tunnel will be wide-reaching: "Having world infrastructure of this nature will add significantly to Limerick's attraction as a place for foreign direct investment," he said.

NRA chairman Peter Malone Chairman, National Roads Authority, said "The Limerick Tunnel will contribute positively to growth in tourism, high end manufacturing and growing business services sector throughout the entire Mid-West."

It is expected that up to 20,000 cars will use the tunnel every day easing the pressure city centre streets and traffic blackspots such as the Condell Road, Ennis Road and the Dock Road.

The construction of the Limerick tunnel was one of the biggest engineering projects ever undertaken in Ireland. It connects the N7 with the N18 and incorporates access points from the N20, N24 and the N69. The road network which facilitates access to the tunnel on each side of the Shannon includes 11 new access bridges, six underpasses and four interchanges.

The tunnel itself, which is 900 metres long, consists of five precast concrete tubes which were made by Austrian specialists, Strabag. They were floated on to the river late last year before they were lowered onto foundations which were set on the river bed.

The public-private-partnership (PPP) Limerick Tunnel, which cost over €600 million to construct, will be operated by Direct Route until 2041 when it will revert into state ownership Tolls range from 90c for motorcycles to €4.50 and €5.70 for large goods vehicles. The price for cars is €1.80.

Motorists who use the tunnel will have pay at one of two toll-plazas, which are located on the Coonagh side of the Shannon. However, motorists can also buy electronic toll-tags which will allow them to pass through the toll-plaza without having to stop.