The River Liffey in Dublin is a major carrier of pollutants into seawater, scientists have warned.
Researchers at Dublin City University say the ongoing use of fossil fuels presents a serious problem.
The study also shows that untreated sewage disposal in the past has led to higher than recommended concentrations of chemicals in the water at Dublin Bay.
The data comes from samples collected at 30 locations in Dublin Bay in June of 2010.
Dr Brian Kelleher from the School of Chemical Sciences in DCU said that any traces of untreated sewage can be dealt with.
"There's still legacy issues there," he said.
"Obviously, what we can do with that sewage is treat it, and that's now being done in a lot of cases, not all cases, in Ireland.
"The other pollutant that we looked at is the effect of using fossil fuels: oil, gas and coal. That is much more difficult because we still want to use these, we still rely on them, humans rely hugely on fossil fuels."