The Environmental Protection Agency said local authorities have boosted coastal cleanliness, but some areas are cleaner than others.
The EPA study investigated the water quality of 148 beaches, which found 78% were deemed excellent, 13% good, 7% sufficient, and just two beaches classed as poor.
The Front Strand in Balbriggan, County Dublin, and Lady's Bay at Buncrana, County Donegal, both received a negative rating from the agency.
The general improvements are a result of enhanced management of bathing waters over many years, combined with investments in treatment of urban waste water.
The increased popularity of sea swimming means more swimmers are in the water outside of bathing season - when water quality isn't monitored.
Swimmers are encouraged to ask their local authorities to officially identify additional local bathing sites. This will ensure they are managed to protect bathers’ health.
Though conditions are improving, the EPA says agriculture, urban waste water and fouling from dogs on beaches still impact water quality.