The Irish Repertory Theatre’s Sean O’Casey Season concludes with The Plough and the Stars, whose title is synonymous with the flag for the Irish Citizen Army. The last of O’Casey’s trilogy, which includes The Shadow of a Gunman and Juno and the Paycock, has joined the other two in repertory, and it’s a rougher, more jagged experience. Like the others, it takes place in a tenement; here the numerous characters move in and out of the parlor of Nora and Jack Clitheroe.