She was born Kathleen Rita McNulty in Feymore, part of the small village of Creeslough in what was then a Gaeltacht area (Irish-speaking region) of County Donegal in Ulster, the northern province in Ireland, on February 12, 1921, during the Irish War of Independence.

[1] On the night of her birth, her father, an Irish Republican Army training officer, was arrested and imprisoned in Derry Gaol for two years as he was a suspected member of the IRA.

On his release, the family emigrated to the United States in October 1924 and settled in the Chestnut Hill section of Philadelphia, Pennsylvania, where he found work as a stonemason.

Their job was to compute ballistics trajectories used for artillery firing tables, mostly using mechanical desk calculators and extremely large sheets of columned paper.

"[10] She and Bilas began work with about 10 other "girls" (as the female computers were called[11]) and 4 men—a group recently brought to the Moore School from Aberdeen Proving Ground.

[7] The two newcomers ultimately learned how to perform the steps of their calculations, accurate to ten decimal places, through practice and the advisement of a respected supervisor, Lila Todd.

[12] A total of about 75 female computers were employed at the Moore School in this period, many of them taking courses from Adele Goldstine, Mary Mauchly, and Mildred Kramer.

[8] After two or three months, Antonelli and Bilas were moved to work on the differential analyser in the basement of the Moore School, the largest and most sophisticated analogue mechanical calculator of the time, of which there were only three in the United States and five or six in the world (all of the others were in Great Britain).

[7] Using the analyser (invented by Vannevar Bush of MIT a decade prior and made more precise with improvements by the Moore School staff), a single trajectory computation—about 40 hours of work on a mechanical desk calculator—could be performed in about 50 minutes.

Later, Antonelli's college schoolmate and fellow computer, Bilas, would join the team of ENIAC programmers at the Moore School, though she did not attend the initial training at Aberdeen.

[16] Her colleague, Jean Jennings, recalled when Antonelli proposed the idea to solve the problem where the logical circuits did not have enough capacity to compute some trajectories.

[citation needed] Following Mauchly's death, Kay carried on the legacy of the ENIAC pioneers by authoring articles, giving talks (frequently along with Jean Bartik, with whom she remained lifelong friends), and making herself available for interviews with reporters and researchers.

[8] In 1997, Antonelli was inducted into the Women in Technology International Hall of Fame, along with the five other ENIAC programmers, for their contributions on programming ballistics trajectories.

The film centered around in-depth interviews of three of the six women programmers, focusing on the commendable patriotic contributions they made during World War II.

[23] In July 2017, Dublin City University (DCU) honoured Antonelli by naming their computing building after Kathleen (Kay) McNulty.

[24] In 2019, the Irish Centre for High-End Computing (ICHEC) at the National University of Ireland, Galway, named its new Waterford-based primary supercomputer, which is to serve as Ireland's national supercomputer for academic researchers, Kay, following a public poll, wherein Antonelli beat out candidates including botanist Ellen Hutchins, scientist and inventor Nicholas Callan, geologist Richard Kirwan, chemist Eva Philbin, and hydrographer Sir Francis Beaufort.