The Irish Sea which separates Great Britain and Ireland has often been written about in terms of divisions, threats, and hazards. Facing each other across that sea, the coast of Wales and Ireland’s eastern seaboard have a more complex story to tell, one characterised by intimate if troubled lines of connection. A place of passage for centuries, the Irish Sea has a distinct identity, its memory longer and more capacious than borders or polities. The region ties together port communities that differ in their stories, cultures, and worldviews. This article explores themes of division and connection across coasts through the story of the Royal Mail Ship (RMS) Leinster, a British mail steamer that was torpedoed just off the coast of Ireland as World War I neared its end and the Irish War of Independence began to stir.