Nano Nagle, who founded the Presentation Sisters in 1777, was given the accolade following a phone-in poll conducted by Marian Finucane's RTÉ radio show.
Sr Nagle was the preferred choice over former president Mary Robinson and disgraced Olympic swimmer Michelle Smith, who provided stiff opposition for the top slot.
Sr Nagle was nominated by Sr Jo Piggott of the North Presentation Convent in Cork.
Sr Piggott, who celebrated her 50th anniversary as a professed sister last October, said she was overjoyed to learn that Nano had topped the poll.
She had nominated Nano for her extensive achievements in the light of Cork being the European Capital of Culture for 2005.
Sr Nagle, who was born into a wealthy family in Ballygriffin, Co Cork, in 1718, became haunted by the plight of disadvantaged women in Cork.
She began her schools with 30 girls in 1755 in a mud cabin of two earthen-floored rooms in Cove Lane in Cork. Other Irish girls, some educated abroad, came to help in her school.
Known as the Lady with the Lamp, Sr Nagle is often pictured as going about in poor unfashionable dress, a lantern in one hand and a stick in the other.
After school hours, Sr Nagle visited the poor, the sick and the dying.
In June 1776 she received the religious habit, the beginning of the Presentation Order. She died in April 1784 at the age of 65.
Over 22,000 people took part in the RTÉ poll, which attracted even more attention when former Olympic triple gold winner Michelle Smith de Bruin emerged as one of the top contenders.
At the end of the three-week poll, however, it was Nano who emerged as the winner with 23.5% of the votes; former president Mary Robinson coming a close second with 21.4%, while Ms Smith got 19.8% of the votes.
In 1998, two years after the games in Atlanta, Georgia, Ms Smith was banned from competitive swimming for four years after being found guilty of tampering with a urine sample.
Her husband and former coach, Erik de Bruin, who nominated her for the title, had served a four-year suspension during his discus career after testing positive for illegal levels of testosterone.
Ms Smith appealed the four-year ban, but it was upheld. After officially announcing her retirement from competitive swimming in 1999, she studied law and will become a barrister next month.
Another surprise was that there was no room at the top for another Cork woman, Sonia O'Sullivan, regarded as the country's greatest athlete. Also absent from the top 10 was Countess Markievicz, Ireland's most famous woman politician, best known for her role in the Easter Rising.
Ms Finucane said the number of people who voted for Michelle Smith was quite astonishing.
"It was said that Michelle Smith was in disgrace but it would appear that three are an awful lot of people that don't accept that," she said.
: Credited with establishing girls' education in Ireland though her work with the Presentation Sisters, which she founded.
: Former President of Ireland who went on to serve as United National High Commissioner for Refugees.
: Won four medals for swimming at the Atlanta Olympic Games in 1996. She was banned from competitive swimming for four years for tampering with a urine sample.
: Also known as Mary of the Gael, she founded a monastery in Co Kildare and was ordained a bishop.
: Sixth century pirate queen.
: Works with street children in Vietnam.
: Legion of Mary envoy.
: Her father Joseph McColgan received the longest ever jail sentence after being convicted of charges relating to the abuse of his children in 1995.
: A 1916 veteran who pioneered the vaccination against tuberculosis (BCG) in Ireland.
: Irish Credit Union pioneer and primary school teacher.