A blood donor is being treated at a Dublin hospital for vCJD, the human form of mad cow disease.
While the man's diagnosis has not been confirmed, his condition is being described as a "probable" case of vCJD based on preliminary test results.
A patient at a second Dublin hospital who received blood donated by the man was informed yesterday that he had received the donation from a man suspected of having vCJD.
The Irish Blood Transfusion Service (IBTS) said last night that the man with probable vCJD had, however, only ever given one blood donation.
It added that components from this donation had only gone to two patients. One of them was dead from a cause unrelated to the blood donation and the other was the patient who was informed yesterday.
It is not known what the risk is of a person contracting vCJD from blood given by a person infected with vCJD, a degenerative brain disease which is invariably fatal.
A spokesman for the National Blood Service in the UK said nobody actually knows what the risks are.
Prof Bill Hall, chairman of the State's vCJD advisory group, said he did not think anybody could quantify the risk. "It could depend on the blood product they received and how much infectious agent was in the blood," he said.
There have been just two cases of vCJD to date in Britain linked to blood transfusion.
The IBTS said that while a person is infected, but symptom-free, it is possible they may pass on the infection through blood donation. "While a number of precautionary measures have been put in place to reduce the risk of transmitting vCJD by blood transfusion in Ireland, no universally effective measure exists to prevent its transmission," it added.
The results of further tests on the man with probable vCJD are awaited. Informed sources said the patient was showing all the signs of having vCJD.
This is the second suspected case of vCJD in the State in the past year. Last October a man in his 20s was treated at a Dublin hospital for the condition. He died recently.
His was the first indigenous case of vCJD in the State and given that he had never given blood or received a blood transfusion, or been operated on, his diagnosis had no adverse implications for the national blood supply. The source of his infection was believed to have been infected meat, which has been the source of the majority of cases of vCJD to date in Britain.
It is considered likely the source of infection in this latest case is also infected meat as blood would not have been taken from him recently if he had ever received a blood transfusion or spent time in Britain when BSE was at its height.
Given that vCJD can be lying dormant for 10 years after eating infected meat, he could have become infected before rigorous food safety controls were introduced here in 1996 and 1997.
Last night Minister for Health Mary Harney said she had been assured that every possible safeguard and preventative measure had been taken by the relevant agencies. "Out of respect for the patient and the patient's family, and the confidentiality of the doctor-patient relationship, the department (of health) is appealing to the press not to identify the hospital and to respect the privacy of the family at this very difficult time," she added.