Behan was born in the inner city of Dublin on 9 February 1923 into an educated working class family. He lived in a house on Russell Street near Mountjoy Squareowned by his grandmother, Christine English, who owned a number of properties in the area. Also living in the area was his uncle Peadar Kearney, song writer and author of the Irish national anthem. Brendan's father, Stephen Behan, a house painter who had been active in the Irish War of Independence, read classic literature to the children at bedtime from sources such as Zola, Galsworthy, and Maupassant; his mother, Kathleen, took them on literary tours of the city. If Behan's interest in literature came from his father, his political beliefs were by his mother. She remained politically active all her life and was a personal friend of the Irish republican Michael Collins. Brendan Behan wrote a lament to Collins, "The Laughing Boy", at the age of thirteen. The title was from the affectionate nickname Mrs. Behan gave to Collins. Kathleen published her autobiography, "Mother of all the Behans," a collaboration with her son Brian, in 1984.
Behan's uncle Peadar Kearney wrote the Irish national anthem Amhrán na bhFiann. His brother, Dominic Behan, was also a renowned songwriter best known for the song "The Patriot Game"; another sibling, Brian Behan, was a prominent radical political activist and public speaker, actor, author, and playwright. Brendan and Brian did not share the same views, especially when the question of politics or nationalism arose. Brendan on his deathbed (presumably in jest) asked Cathal Goulding, then the Chief of Staff of the IRA, to 'have that bastard Brian shot—we've had all sorts in our family, but never a traitor!'.
A biographer Ulick O'Connor, recounts that one day, at the age of eight, Brendan was returning home with his granny and a crony from a drinking session. A passer-by remarked, "Oh, my! Isn't it terrible ma'am to see such a beautiful child deformed?" "How dare you", said his granny. "He's not deformed, he's just drunk!"
Behan left school at 13 to follow in his father's footsteps as a house painter.
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