Public notices advising bathers not to swim have been erected at Grattan Beach in Salthill, Co Galway.
It comes after recent bathing water samples showed levels of E coli that breached EU mandatory permitted values.
Excessive rainfall over the past week is said to be a major contributory factor to the increased levels of bacteria detected.
Galway City Council is to continue monitoring the water quality at all beaches and took further samples today, with results known tomorrow.
Meanwhile, Cork County Council has said the levels of E coli at seven Cork beaches have significantly decreased.
Public notices advising against swimming at Youghal, Claycastle, Redbarn, Coolmain, Garretstown, Oysterhaven and Garryvoe were first posted last week.
The council is awaiting the outcome of discussions with the Health Service Executive in relation to the swimming notices.
