On 8 March 1918, Markievicz was arrested under the Defence of the Realm Act and imprisoned in Holloway Prison, London, for alleged involvement in a pro-German plot.
On 8 March 1918, British authorities arrested Constance Markievicz along with numerous other Sinn Féin leaders under the Defence of the Realm Act, accusing them of involvement in the so-called 'German Plot' — an alleged conspiracy between Irish republicans and Germany during the First World War. She was transported to Holloway Prison in London, a facility notorious for holding political prisoners, including suffragettes.
The arrests were widely viewed in Ireland as a politically motivated crackdown intended to weaken Sinn Féin ahead of anticipated elections, rather than a response to genuine evidence of collusion with Germany. Far from suppressing support for the movement, the imprisonments generated public outrage and further galvanised nationalist sentiment across Ireland.
Remarkably, Markievicz's imprisonment did not prevent her from standing as a candidate in the December 1918 general election; she campaigned, in effect, from her prison cell. Her continued incarceration during this pivotal contest only enhanced her standing as a committed and unyielding revolutionary figure, and her subsequent election victory while still behind bars became one of the most symbolically potent moments of the Irish independence movement.