Radon levels in a house in Castleisland, Co Kerry, are over 200 times the national reference level and the amount of radiation in the house in a day was what a nuclear plant worker might experience in a year.
A husband and wife in the house, both non-smokers, developed lung cancer. The woman later died from the disease. Eight of their neighbours also developed cancer.
A spokesman for the Radiological Protection Institute of Ireland warned against the "apathy" which surrounds radon, a naturally-occurring radioactive gas from the soil.
Radon levels of 50,000 becquerels, some 250 times higher than the national reference level of 200 becquerels per cubic metre, were recorded in the house.
Mr Tony Colgan, principal scientific officer with the Radiological Protection Institute of Ireland (RPII), said such levels meant people in the house would be subjected to the same kind of levels of radiation in a day as a worker in the nuclear energy plant in Sellafield might experience in a year.
The RPII wrote last weekend to some 2,500 householders in the Castleisland area asking them to have radon levels measured in their homes. The RPII can carry out monitoring and the name of another company was also provided, along with an information leaflet.
So far there has been a very poor response, Mr Colgan said.
It is well known that the Tralee and Castleisland areas are high risk areas for some time, he said.
Some years ago high levels were also found in a house in south Wicklow and several hundred householders were written to.
The response from householders there too was negligible, Mr Colgan said.
"There's a great apathy out there about radon," he added.
The RPII was not trying to frighten anyone but people living in houses with radon recordings significantly above the national reference level had a one-in-four chance of developing lung cancer even if non-smokers.
Some 91,000 homes are estimated to have radon levels above the national reference level and of these 7 per cent have dangerously high levels.
"If high radon levels are found in houses, something can be done about it," Mr Colgan stressed.
There was concern in Castleisland about the levels of radon, local GP, Dr Eamonn Shanahan said. However, there had been no obvious on-going clusters of cancer in the area.
Since July, 1998, building regulations require new houses in high radon areas to have a fan fitted to extract the gas.