Published Wednesday, 27 May 2009
The Irish Department of Health said the patient is a woman from the east of the country who recently travelled from New York.
"She is aware of the confirmed test result and is currently being treated at home and all necessary public health protection measures are being taken," said a spokeswoman.
Officials said the latest confirmation of the virus is unconnected to the second case which was announced on Monday.
That patient was also a woman from the east of the country who recently returned from New York.
Chief Medical Officer Dr Tony Holohan added: "We would also ask that people who have visited an affected area and become ill with flu like symptoms make their initial contact with their GP by telephone and not present at a GP surgery or A&E unit."
Last week the Irish government lifted a ban on non-essential travel to Mexico as officials believed the risk of infection had receded.
Information leaflets on swine flu were also distributed to two million homes in recent days.
The Health Service Executive (HSE) is currently involved in talks with pharmaceutical firms to stockpile vaccines for the virus.