Fingal County Council have urged the public not to swim at two beaches in the area.
The warning comes as the council recently imposed bans at Loughshinny and Rush South after testing found high levels of bacteria in the water.
However the council do say the bans are temporary and they will be lifted when levels return to normal.
It is believed that sewage from a wastewater treatment plant or sewer network is the reason why the water is contaminated.