In 1976 Dr Patricia Sheehan, a Dublin doctor, was talking to a mother who had a Down syndrome child. Down syndrome, a disorder in which there are three chromosomes number 21 instead of the usual two, is also called trisomy 21. It causes changes in appearance which gave the disorder its old name, ‘mongolism’, and usually causes mental retardation. The mother told Dr Sheehan that there were other girls who had attended her school, St Louis, in Dundalk, Co. Louth, in 1956/57 who also had Down syndrome babies and that the mothers were young when the babies were born.

Patricia Sheehan decided to investigate the story and spent the next eighteen years finding out what had happened to the 178 girls who were attending the school at that time. She was able to trace 159 of them, 89 per cent, and questionnaires were completed by them and returned. She found six of the mothers had given birth to Down syndrome babies. Patricia Sheehan and a microbiologist, Professor Irene Hillary, published an account about this cluster in the British Medical Journal in 1983. In a follow-up letter, the number of girls at the school in 1956/57 reported to have had Down syndrome babies increased to eight.

In October 1957 there was a fire at Windscale, now Sellafield, the nuclear power and reprocessing station which is across the Irish Sea from Dundalk. At about the same time as the fire, there was also an influenza epidemic which affected many people in Dundalk, including girls at the school. The question was raised: was radiation from the fire at Windscale responsible for damage to the ova of some of the children attending the school, which had caused them to give birth to a child with an extra chromosome 21.

After the Chernobyl disaster in 1986 there was greatly increased concern about the possible effects of radiation on health. This was made more acute by the reported increase in leukaemia in Cumbria around the Sellafield site. In Ireland it was feared that not only Down syndrome, but various forms of cancer and other disorders might be due to radioactive material escaping from the reprocessing plant in Cumbria.