To what does the title of Sean O'Casey's The Plough and the Stars (1926) refer?

The title of Sean O’Casey’s The Plough and the Stars refers to the banner of the Irish Citizens Army, of which O’Casey was once a member.

The play concerns members of the army before and during the Easter Rising in 1916.

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To what does the title of Sean O’Casey’s The Plough and the Stars (1926) refer?