Seán O'Casey born John Casey, 30 March 1880 – 18 September 1964) was an Irish dramatist and memoirist. A committed socialist, he was the first Irish playwright of note to write about the Dublin working classes.

Biography
Early life
O'Casey was born in Dublin, Ireland, as John Casey or John Cassidy to Michael and Susan Archer Casey in a house at 85 Upper Dorset Street, in the northern inner-city area of Dublin. It is commonly thought that he grew up in the working class society in which many of his plays are set. In fact, his family were considered as "shabby genteel". He was a member of the Church of Ireland, baptised on July 28, 1880 in St. Mary's parish, confirmed at St John the Baptist Church in Clontarf, and an active member of Saint Barnabas until his mid-twenties, when he drifted away from the church.
 
O'Casey's father died when Seán was just six years of age, leaving a family of thirteen. The family lived a peripatetic life thereafter, moving from house to house around north Dublin. As a child, he suffered from poor eyesight, which interfered somewhat with his early education, but O'Casey taught himself to read and write by the age of thirteen.
 
He left school at fourteen and worked at a variety of jobs, including a nine-year period as a railwayman. O'Casey worked in Easons for a short while, in the newspaper distribution business, but was sacked for not taking off his cap when collecting his wage packet.
 
From the early 1890s, O'Casey and his older brother, Archie, put on performances of plays by Dion Boucicault and William Shakespeare in the family home. He also got a small part in Boucicault's The Shaughraun in the Mechanics' Theatre, which stood on what was to be the site of the Abbey Theatre.
 
Politics
As his interest in the Irish nationalist cause grew, O'Casey joined the Gaelic League in 1906 and learned the Irish language. At this time, he Gaelicized his name from John Casey to Seán Ó Cathasaigh. He also learned to play the Uilleann pipes and was a founder and secretary of the St. Laurence O'Toole Pipe Band. He joined the Irish Republican Brotherhood, and became involved in the Irish Transport and General Workers Union, which had been established by Jim Larkin to represent the interests of the unskilled labourers who inhabited the Dublin tenements. He participated in the Dublin Lock-out, but was blackballed and could not find steady work for some time.
 
In March 1914 he became General Secretary of Larkin's Irish Citizen Army, which would soon be run by James Connolly. On 24 July 1914 he resigned from the ICA, after his proposal to deny dual membership to both the ICA and the Irish Volunteers was rejected.

In 1959 O'Casey gave his blessing to a musical adaptation of Juno and the Paycock by American composer Marc Blitzstein. The musical, retitled Juno, was a commercial failure, closing after only 16 Broadway performances. It was also panned by some critics as being too "dark" to be an appropriate musical, a genre then almost invariably associated with light comedy. However, the music, which survives in a cast album made before the show opened, has since been regarded as some of Blitzstein's best work. Although endorsed by the now 79 year old O'Casey, he did not contribute to the production or even see it during the brief run. Despite general agreement on the brilliance of the underlying material, the musical has defied all efforts to mount any successful revival.
 
Also in 1959, George Devine produced Cock-a-Doodle Dandy with at the Royal Court Theatre and it was also successful at the Edinburgh International Festival and had a West End run.
 
In 1960 was his eightieth birthday, and to celebrate, David Krause and Robert Hogan wrote full-length studies. The Mermaid Theatre in London launched the "O'Casey Festival" in 1962, which in turn made more theatre establishments put on his works, mostly in England and Germany. It is in these late years that O'Casey put his creative energy into his six-volume Autobiography.
 
In September 1964 at the age of 84, O'Casey died of a heart attack, in Torquay, England. He was cremated at the Golders Green Crematorium.
 
In 1965, his autobiography Mirror in my House (the umbrella title under which the six autobiographies he published from 1939 to 1956 were republished, in two large volumes, in 1956) was turned into a film based on his life called Young Cassidy. The film was directed by Jack Cardiff (and John Ford) featuring Rod Taylor (as O'Casey), Flora Robson, Maggie Smith, Julie Christie, Edith Evans and Michael Redgrave.

Personal life
He was married to Irish actress Eileen Carey Reynolds (1903–1995) from 1927 to his death. The couple had three children: two sons, Breon and Níall (who died in 1957 of leukemia), and a daughter, Siobhán.
 
Archival collection
In 2005, David H. Greene donated a collection of letters he received from O'Casey from 1944 to 1962 to the Fales Library at New York University. Also in the collection are two letters written by Eileen O'Casey and one letter addressed to Catherine Greene, David Greene's spouse.
 
O'Casey's papers are held in the New York Public Library, the Cornell University Library, the University of California, Los Angeles Library System, the University of London Library, the National Library of Ireland and the Fales Library.