Casey was one of a group of Fenians arrested in 1865 in Cork and transported to Western Australia with other Fenians captured in the abortive 1867 Rising. "A Mingling of Swans" includes Casey's unpublished account of his experiences as a convict on roadwork parties, as well as correspondence by Casey and other Fenians, and some articles by Casey on his impressions of Western Australia which were published in Dublin separatist newspapers. Casey portrays with humour and determination the harsh conditions endured by Fenian prisoners.

This very vivid memoir describes the prison experiences of a Cork Fenian activist, John Sarsfield Casey. 'The Galtee Boy' was a name used by Casey when he sent letters for publication to newspapers, one of which was used against him at his trial in 1865. His memoir was written after he had returned from deportation and describes the period from his arrest in 1865, his trial in Cork and conditions in Mountjoy, Millbank, Pentonville and Portland prisons. His memoir is the most extensive surviving account from the Fenian side of the experiences of those prisoners detained in Cork. Biographies of people mentioned in the memoir are given in an appendix.