The source of a recent outbreak of gastroenteritis in the midlands is the private water supply of a Co Offaly hotel, but investigators do not yet know what organism caused the infection.
This water supply is no longer being used, the Midland Health Board and hotel management stressed.
The Food Safety Authority of Ireland (FSAI), which was involved in the investigation, said the potentially fatal E.coli 0157 bacterium was not involved.
Last month, the Cistercian College, Roscrea, Co Tipperary, and the County Arms Hotel, Birr, Co Offaly, closed temporarily due to outbreaks of the infection.
The hotel, which closed voluntarily when the outbreak was confirmed, was shut for two weeks.
Three students from the college developed symptoms of the infection.
The school was closed for 48 hours and boarding students sent home.
The Midland Health Board said all food premises had to have an adequate supply of water which met EU food hygiene regulations, including chemical and microbiological criteria.
"In this case, the microbiological criteria of a private water supply were breached with an excess of indicator organisms, i.e. coliforms, thus indicating contamination."
Dr Margaret Fitzgerald, chief specialist in public health at the FSAI, said coliforms were an indication of faecal contamination.
She stressed, however, that if a premises had an unsafe water supply, there was potential for an E.coli 0157 outbreak.
Mr William Loughnane, managing director of the hotel, said the private water supply from a well had been disconnected, and it was now using the urban supply.