People are being told not to try to walk or swim across Baldoyle Bay, a notorious blackspot for dangerous currents along with sinking sand.
Last Friday a teenage boy had to be rescued after clinging to a buoy in the area for half an hour.
Two other boys have also been reported getting into difficulty at Cush Point. All three of them emerged unharmed.
It's really dangerous.
Social Democrats councillor Joan Hopkins said it has become more of an issue as weather improved in the last couple of months, with more visitors at beaches.
Ms Hopkins said three beaches at Portmarnock, Sutton and Baldoyle meet to form Baldoyle Bay.
"They meet and when the tide is out it looks like it's easy to cross over, it really isn't, it's really, really dangerous," she said.
Ms Hopkins added: "I’ve ordered additional signage from Fingal County Council for Sutton, Portmarnock and Baldoyle. Burrow Beach in Sutton and Baldoyle Estuary have very strong currents. You cannot cross at low tide on foot from one beach to another, it’s dangerous."
Ms Hopkins' request to Fingal County Council to have warning signs installed at the beaches has been approved by the local authority.
