Margaret Hegarty was born on 11 April 1880 in Skibbereen, County Cork, Ireland (then part of the United Kingdom). While she later claimed to be born on the 25th of March, her birth record indicates she was actually born a week later. She married Daniel Hegarty, who coincidentally shared her surname. She moved to the United States in the early 20th century, and began working as a housekeeper. She was employed at the St. Augustine Church rectory in Oakland, California for around 30 years before retiring in 1953. Following her husband’s death, she also worked as a cook for schools and churches throughout California.

Around 1970, she moved into St. Anne’s Home, a retirement home for low-income elderly people ran by Little Sisters of the Poor in San Francisco. At the time of her 100th birthday in 1980, she had outlived her three children, but was visited by her son-in-law, four grandchildren, and great-granddaughter. Her birthday was celebrated with a St. Patrick’s Day-themed party, and was serenaded by an Irish singer and bagpipe player.

Margaret Hegarty died in San Francisco on 14 November 1991, at the age of 111. Her record as the oldest Irish-American person would stand for over 20 years, until being surpassed by Kathleen Snavely on 1 October 2013.