The Molly Maguires were a secret militant organization of Irish miners working in the Pennsylvania anthracite coal mines in the 19th century.
They organized in 1854 to fight the mine operators. In 1875, a Pinkerton spy working for the owners infiltrated the group.
That led to the conviction and hanging of 20 Molly Maguires on charges of blackmail and death threats against mine officials.
With a number of other Molly Maguires imprisoned, the organization fell apart.