Catherine Flannagan was born in Ireland around 1829. During the devastating Irish Potato Famine, she, like many others, migrated to Liverpool, England, in search of better opportunities. By 1880, Catherine operated a boarding house at 5 Skirving Street, Liverpool, where she lived with her sister, Margaret, and other tenants. Despite their efforts, financial struggles plagued the household, leading Catherine and her sister to devise a lethal scheme to exploit burial societies—organizations that provided financial assistance for funerals upon a member's death.

The sisters' deadly plan began with the sudden death of Catherine's 22-year-old son, John Flannagan, in December 1880. His passing, though abrupt, did not initially arouse suspicion, and Catherine collected £71 from his burial society—a substantial sum at the time. This success emboldened the sisters to continue their scheme. Over the next few years, several other individuals associated with the household died under mysterious circumstances, including Margaret Jennings, a 16-year-old lodger, in January 1883. In each case, Catherine and Margaret benefited financially from the burial society payouts.

The turning point came in September 1883 with the death of Thomas Higgins, Margaret's husband. His sudden illness and death raised suspicions, leading to a post-mortem examination that revealed lethal amounts of arsenic in his system. Subsequent investigations uncovered arsenic in the sisters' residence, and both women were arrested and charged with murder. Catherine attempted to evade capture but was apprehended ten days later.

In February 1884, after a three-day trial, Catherine and Margaret were found guilty of the murder of Thomas Higgins. Despite suspicions of additional murders, they were only tried for this single case. On March 3, 1884, the sisters were executed by hanging at Kirkdale Prison in Liverpool.