An Arsenal FC supporter, she spent one season with Esker Celtic playing in the Dublin and District Schoolboys'/girls' League (DDSL) before moving to Peamount United.
[3][4] In September 2015 Farrelly made her debut for Peamount United's first team in a 4–2 WNL Shield win over Castlebar Celtic.
Spurred on by that rejection, she missed her Debs to train with the Republic of Ireland women's national under-19 football team and was then appointed captain at the higher age group.
[21] In August 2016 Ireland coach Susan Ronan named Farrelly in a young and predominantly home-based senior squad for a training camp in Wales.
[24][25] Ronan's successor as national team coach Colin Bell called up Farrelly for the first time as a late replacement in the FIFA Women's World Cup qualifying fixture away to European Champions the Netherlands in November 2017.
[26] In August 2019 she started a 3–0 friendly defeat by the United States at the Rose Bowl in Pasadena, California, and was complimented on her performance by interim manager Tom O'Connor: "Niamh Farrelly and Jess Gargan were excellent, they didn't look out of sync at all".