Committed to the Irish nationalist cause, she delivered the surgical knives used in the assassinations of the Permanent Under Secretary Thomas Henry Burke and the newly installed Chief Secretary for Ireland, Lord Frederick Cavendish (the Phoenix Park Murders), to the Irish National Invincibles in February 1882 when she was seven months pregnant, concealing the knives under her skirts.

[1][2] The evidence of James Carey implicated Byrne in the Phoenix Park Murders, leading to her arrest at her home on Avondale Road, Peckham Rye, south London in February 1883.

At a meeting in New York to honour the executed Invincibles in May 1885, she was presented with a "well-filled purse" and pronounced a "brave little woman" who was "as true as steel to all those heroic ideas of womanhood which typify the feminine character of Ireland."

She became a member of an American ladies’ committee which erected a monument to Patrick O’Donnell, executed for killing James Carey, in Glasnevin in April 1887.

Believing she was near death, she told an American journalist that Parnell was unconnected to the Invincibles in June 1894.