Radon is a gas from uranium in the ground. You cannot smell it. It gets into houses through floors and builds up in sealed rooms, and the EPA puts about 350 lung cancer cases a year in Ireland down to it. It is also one of the few risks with a clear map and a cheap fix, which makes it a line item rather than a reason to walk away.
The map
The EPA radon map divides the country into grid squares and predicts, from measurements, the share of homes in each square above 200 becquerels per cubic metre. A square where 1 in 5 or more are predicted above is a High Radon Area. Large parts of Galway, Mayo, Kerry, Cork, Wicklow and Carlow are in it. Dublin mostly is not. It follows the rock, so it can change between two towns ten kilometres apart. We read the map at the centre of a thousand towns: radon by county and town.
The 1998 rule
Since July 1998 new houses in High Radon Areas must have a radon barrier under the floor, and every new house must have a standby sump. So a 2005 house in Tralee should have a barrier; a 1985 one will not. Ask for the certificate of compliance if the house is post-1998. Barriers fail if they were badly laid, so a test is still worth doing.
Testing and fixing
The test is two detectors left in place for three months, then posted to a lab. About €50 to €60 from the EPA or a registered lab. You cannot do this before you own the house, which is why the question to the seller matters: was it ever tested, and what was the result. If the answer is a number above 200, ask what was done. Fixing an existing house usually means a sump and fan (€1,500 to €3,000) or improved ventilation for marginal cases.
The report reads the EPA prediction for the exact Eircode square and checks the build year against the barrier rule, then puts it beside flood, BER and the other checks so you see which one actually matters for that house.