A signalling fault at Dublin Port Tunnel is blamed for this morning's traffic congestion in north Dublin.
Motorists were delayed for one hour as the situation was made worse by a series of traffic accidents.
The National Roads Authority says a height detector tripped on the southbound entrance and turned the traffic signal red just before 6.20am.
It then stayed red because of what is described as a computer problem and took over two and half hours to fix while the southbound tunnel section remained closed.
Gardaí took over traffic management and lifted the HGV cordon for trucks going into the city centre.
But the M1 and connections to the Malahide Road were quickly congested.
Two crashes outside the tunnel entrance and another in the city centre made things worse and traffic did not clear until after 9am.
In the Dáil this morning the Taoiseach Bertie Ahern dismissed Opposition complaints by saying this was the first problem with the tunnel since Christmas.
The Automobile Association said the roads authority should examine faster ways of repairing its safety systems.
