Complex sentence: The Molly Maguires was an Irish 19th century secret society active in Ireland, Liverpool and parts of the eastern United States, best known for their activism amongst Irish American coal miners in Pennsylvania.
Split into simpler sentences: The Molly Maguires was an Irish 19th century secret society active in Ireland, Liverpool and United States. The "Mollies" are mostly known for their activism amongst Irish American coal miners in Pennsylvania.