Historians for decades have placed Room 40, the First World War British naval signals intelligence organization, at the centre of narratives about the British anticipation of and response to the Easter Rising in Ireland in 1916. A series of crucial decrypts of telegrams between the German embassy in Washington and Berlin, it has been believed, provided significant advance intelligence about the Rising before it took place. This article upends previous accounts by demonstrating that Room 40 possessed far less advance knowledge about the Rising than has been believed, with most of the supposedly key decrypts not being generated until months after the Rising had taken place.