The Minister for Transport, Mr Brennan, is in favour of changing the height of the Port Tunnel if it can be done at a reasonable cost and without creating serious delays for the project.
Mr Brennan is awaiting more information from consultants on whether the changes can be done at a cost of €30 million or less.
According to officials at his Department, the Minister has also asked for specific details on the time-frame of the lower-cost options.
Under the current height restrictions, large "supercube" lorries will be banned from the tunnel.
However, senior staff at Dublin City Council, which is overseeing the €650 million project, and the National Roads Authority are opposed to any height change. They believe any such change would cost a minimum of 70 million, and delay the opening of the tunnel by 12 months.
Mr Brennan has to make his decision within three months, by which stage the fitting out of the tunnel is due to begin. Staff at the Department expect him to make a decision in the next month.
Meanwhile, Dublin City Council is proposing to ban heavy goods vehicles (HGVs) from the inner city during daytime hours once the port tunnel comes into operation in August 2005.
In its draft HGV Management Strategy, published yesterday, the council said trucks with three or more axles or a gross weight of more than 17 tonnes would be banned from using streets within the canal cordon between 7 a.m. and 7 p.m.
Provision is being made for permits to be issued to cover a number of exceptions. The extent to which exceptions will be made, such as for deliveries to city centre shops, is to be decided after a public consultation process, starting today.
The main objectives of the strategy are to ensure that the full potential of the Dublin Port Tunnel is realised in terms of removing port-related HGV traffic from the city, and to reduce the volume of non-port related HGV traffic in the city centre.
Last year Mr Brennan commissioned a report from independent consultants Atkins following complaints from transport and business lobby groups about the height of the tunnel.
The consultants reported to him last October, outlining a range of options carrying costs of between 30 million and 100 million.
However, the Minister was not satisfied that there was sufficient detail on the exact costs and time-frames associated with some of the cheaper options.
Yesterday the NRA said it was prepared to introduce high toll charges at certain peak times on the Port Tunnel in order to reduce the volume of traffic using it at peak times.
The authority has already set a higher toll of 5.60 for vehicles travelling south into the city during the morning rush hour.
The normal toll will be 1.70 at all other times for traffic travelling north or south.
An NRA spokesman said the 5.60 toll had been set to discourage drivers from using the tunnel "as a fast access into the city centre".
Meanwhile, the Department of Transport has begun consultations with various interest groups in relation to "supercube" trucks, with a view to banning them on Irish roads. Ireland and Britain are the only two EU countries to allow the large trucks.
A spokesman for Mr Brennan said the fact the Government was considering a ban would have no bearing on the Minister's final decision.