Part of Dun Laoghaire's East Pier is at risk of structural failure as its support beams succumb to corrosion, a new report has found.
Repairs to a 75 metre section of the pier will cost up to €5m.
The unpublished report, commissioned by Dun Laoghaire Harbour Company, details how structural beams which support an area of the pier known as Berth 1 are at risk of failure.
"Steel reinforcing has reacted over the years. It has been open to the sea for a long time," said Dun Laoghaire Harbour Master, Simon Coate.
"That needs repairing and there is a process to do that by breaking the old concrete and putting in new reinforcing."
But Mr Coate stressed that the platform above the support structure is safe.
"It can't take really heavy loads and that's why we have put bollards along to stop heavy loads going on it," he added.
'Berth 1' is a 75 metre stretch of concrete on the left of the original East Pier before the bandstand.
It was added after the original construction to facilitate boats mooring alongside the pier itself which was built almost 200 years ago and is enjoyed by more than one million people a year.
The Harbour Company said the pier will not need to close while repair work is undertaken.
Because the berth is beside the proposed urban beach, which was granted planning permission last week, the Harbour Company has said repair work on it will dovetail with the construction of the new amenity, which is scheduled to open in 2016.
The cost of the repairs is estimated by the engineers report to be between €1m and €5m.
Local Fine Gael Councillor Barry Ward called for the work to be carried out as a matter of urgency.
"It is not dangerous to people now, but how long will it take before it becomes dangerous? The worst-case scenario is that there is structural failure and a part of it breaks," Cllr Ward warned.
"In many ways the harbour defines Dun Laoghaire. It's a tremendously important commercial and leisure amenity," he added.