The Jack Lynch Tunnel in Cork is to close on weeknights for up to six months from next October for work to bring it in line with European Union fire safety standards.
The closure has been confirmed by the National Roads Authority and the work is expected to cost in the region of €5m.
The NRA is proposing to carry out the work on the tunnel over a six-month period from next October.
The Jack Lynch Tunnel was opened in 1999, 21 years after it was first conceived.
At the time it was the biggest infrastructural project ever undertaken by a local authority, with the 610m twin-bore tunnel and 1.8km of approach roads costing just over IR£100m.
The tunnel was built to accommodate traffic of 25,000 vehicles initially, eventually rising to 40,000.
In fact, average daily throughput now exceeds 65,000 vehicles and the tunnel has become vital for the free movement of traffic in and around Cork city.
New EU regulations on fire safety mean both the north and south bores of the tunnel have to be retro-fitted with flame-retardant cladding to protect the structure in the event of a major fire.
The Dublin Port Tunnel and the Limerick Tunnel under the River Shannon already comply with these regulations.
The NRA says the work is being done at night to minimise disruption and the closures will be suspended during major events in the city.
