Four beaches, including Rush in North Co Dublin, failed to comply with minimum mandatory standards and were classified as having 'poor' water quality.
The others were Clifden Beach in Galway, Fountainstown in Cork and Ballyheigue in Kerry.
Some 91 of 136 designated bathing areas were classified as having 'good' water quality last year. The figure was down from 112 out of 135 in 2011.
Among those downgraded to 'sufficient' were Skerries South Beach, Merrion Strand and Dollymount Strand, all in Dublin.
"The drop in the numbers of waters achieving 'good' status is disappointing, but is clearly linked to the weather," Peter Webster, senior scientific officer with the Environmental Protection Agency (EPA), said.
The data were released in the EPA's report, The Quality of Bathing Water in Ireland – An Overview for the Year 2012.
However, EPA's Laura Burke, said: "Irish bathing waters continue to be among the best in northern Europe."
New stricter standards are to take effect from 2014.