Radioactive iodine from the tsunami-hit Fukushima nuclear plant in Japan is believed to have reached Northern Ireland.
Authorities said it is believed radiation levels will be consistent with the trace levels reported across Europe in the wake of the nuclear disaster.
Detection equipment in Dublin found radioactive iodine in the atmosphere known to have come from the stricken reactor.
Health authorities in the UK and Ireland said the levels are too low to pose any threat to public health. They said the dose from breathing in air falls well below background radiation levels.
David Dawson of the Radiological Protection Institute of Ireland (RPII) said the amount discovered was so small it would have no impact on people.
He said 20 micro-becquerels had been picked up by the high-volume air sampler, but the levels would have to be in excess of thousands of becquerels to spark health fears.
Radioactivity is dispersing from Japan across the northern hemisphere and diluting as it migrates eastwards, he added.
Traces of radioactivity from the nuclear disaster have also been picked up by monitoring stations in Oxfordshire and Glasgow. The Oxfordshire station measured 300 micro-becquerels per cubic metre, much less than the natural background dose to which a person in the UK is likely to be exposed in normal circumstances.
The Department of the Environment said the levels of radioactive iodine detected in Scotland were very low and of no health concern.
“There are five continuous gamma dose rate monitors in Northern Ireland, but the contribution that this concentration of iodine would make is indistinguishable from background and hence would not be picked up,” a spokesman said.
Northern Ireland Green Party leader Steven Agnew said: “The issue of the Fukushima reactor has brought nuclear power to the fore and brought safety concerns back onto the agenda.”
Friends of the Earth activist Declan Allison said the news meant the UK must rethink plans to build more reactors.