The IBTS said the infection occurred after the blood donation was made in the "window period" when the virus can not be detected, even by the most sensitive tests available.
In the case, a blood donation was taken and the red cells were transfused to a patient, as a standard medical procedure.
This afternoon, the Irish Blood Transfusion Service (IBTS) confirmed to Newstalk that the patient has since made a full recovery and is now clear of the infection. The blood service said that it "does not have any implications for blood that will be transfused to patients in the future".
An investigation carried out by Grifols/Hologic, the manufacturer of the assay used to test for hepatitis B virus (HBV), confirmed the assay performed to the expected standard of sensitivity for the detection of HBV, and therefore the result is not due to a test failure.
Speaking on RTÉ's News At One, Dr Stephen Field, IBTS Medical and Scientific Director, said there is a period of a "about 15" days, where infected blood can pass as negative.
It said the donation was made in the "window period when the presence of the virus can not be detected even by the most sensitive tests available".
The patient is now being managed by a medical team.
At the time the donation was tested.
Transmission by transfusion is now rarely encountered due to highly sensitive laboratory testing of each blood donation.
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An archived sample of the blood donation was tested again and it tested negative. "I am sure the blood transfusion service will continue to keep the public and indeed me as minister up to date".
An extra test for Hepatitis B virus DNA was positive when tested.
Dr Field has said the blood service had a responsibility to preserve the identity of the patient and the blood donor.
An estimated 257 million people are living with Hepatitis B. However, it's uncommon in Ireland compared to many countries and blood donors are among those with the lowest prevalence of the virus.
Hepatitis B virus is transmitted through exposure to infected blood and body fluids such as semen and vaginal secretions.
While treatment is available for those who become chronically infected, vaccination and screening is key in preventing spread to others.
Responding to the incident today, Minister for Health Simon Harris said that as far as he's aware, this was the first instance of this in the country.
He said this is an extraordinarily rare incident.
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