An RPII survey revealed around 10% of homes built after July, 1998, which should contain radon barriers, had gas levels above the national recommendations.
One recently-built house was recorded with radon over seven times the national safety level of 200 becquerels per cubic metre.
The naturally occurring radioactive gas is believed to be responsible for around 200 deaths from lung cancer each year.
However, the survey on homes in Kilkenny also showed that radon levels in homes built after 1998 - which are required to be equipped with radon preventative measures under building regulations - fell by 33% on average.
Radon concentrations above the national reference level were found in just under 6% of houses built between 1992 and 1997.
RPII spokesperson David Fenton said: "The only proper precaution for any homes is to ensure it is tested for radon after it has been occupied."
As a result, the RPII is advising all householders to have tests which cost around an average €45 conducted on their homes for the gas.
Since July, 1998, all new homes in high radon areas are required to be fitted with a radon barrier that consists of a special sheeting which prevents the odourless, colourless gas from entering a building. An RPII spokesperson said it had not been determined if the Kilkenny home with dangerous radon levels had been equipped with such a barrier or a radon sump.
However, he stressed that radon-reducing methods will ensure levels can be brought down to recommended safety levels.
The RPII also encourages people to carry out further readings for radon levels after equipment has been installed.
The owners of homes in Kilkenny with dangerous levels of radon have been notified.
According to the RPII, over 250,000 people could be living unknowingly in homes with dangerous levels of the naturally occurring radioactive gas. The RPII estimates that around 91,000 households have radon levels above the recommended safety levels.
However, an RPII spokesperson said the institute had so far discovered only 3,000 such homes.
It means that around a quarter of a million people may be unaware that their residences are located in areas with dangerous levels of exposure to radon.
Only 58% of house-holders are actually aware that their home is equipped with radon-reducing equipment, while 24% were unaware that no radon barriers were fitted to their homes.
The RPII said it was also encouraging all parties involved in building conveyancing to highlight the need for all houses to have measurements for radon levels taken.
. Radon is a naturally occurring radioactive gas derived from the decay of uranium in the ground.
Yes, but only if it enters an enclosed space such as a building as it is known to have positive links with lung cancer. It is estimated up to 200 deaths a year in Ireland are due to radon exposure.
Radon is an odourless, colourless gas. It can only be measured using special equipment. The Radiological Protection Institute of Ireland recommends that all homes be tested.
One detector is placed in a bedroom and another in a living room with readings taken over a three-month period.
Every county in Ireland has areas with radon above the recommended safety level. Large areas with known high concentrations of radon are found along the west coast and in the South-East.
Yes. Building regulations introduced in July 1988 require all new homes to have equipment to extract radon, such as a sump.
The Radiological Protection Institute provides information on radon on its website at www.rpii.ie