Swimmers are being warned to stay out of the sea for at least 48 hours after heavy rainfall, due to pollution fears.
The Environmental Protection Agency says it applies to all beaches at all times.
It comes as the agency publishes its latest bathing water quality report.
The report found four bathing areas - at Clifden Beach in Galway, Lilliput in Westmeath, Cúas Crom in Kerry and Balbriggan Beach in Dublin - failed to meet minimum standards last year.
However, it says water quality is continuing to improve overall - with 111 out of 148 bathing waters deemed "excellent" in 2020, an increase of four compared to the previous year.
In total, 96% of Ireland's bathing waters are meeting the EU's minimum standards.
Two beaches with "poor" water quality in 2019 have improved - Ballyloughane Beach in Co Galway and Brook Beach in Portrane, Dublin.
Meanwhile, two new bathing waters - Carrigaholt and Quilty, both in County Clare - were identified last year.
Dublin's Merrion Strand is no longer considered a swimming spot due to persistently poor water quality.
Sea swimming
The EPA's Mary Gurrie says water sport enthusiasts and sea swimmers should be careful after rain at all bathing spots.
She told Newstalk Breakfast: "The water quality can change in the short-term, particularly after heavy rainfall. People need to be aware of that.
"As a rule of thumb: if there has been heavy rain and they do swim regularly, the water quality may not be as good as it usually is. Keep that in mind."
However, she said the quality of Irish bathing waters is overall a "good news story".
She said if a beach has 'excellent', 'good' or 'sufficient' water quality it is safe to swim in.
Water quality is monitored during the bathing season, which is between June and mid-September.
However, with the recent rise in popularity of sea swimming all-year-round, there have been calls for the monitoring period to be increased.
Ms Gurrie said the EPA also believes there's now a need for "better information and more information" throughout the year.
The latest updates on water quality can be found on the EPA's dedicated Beaches.ie website.
