Sometime in 1915, Plunkett joined the
Irish Republican Brotherhood and soon after was sent to Germany to meet with
Roger Casement, who was negotiating with the German government on behalf of Ireland. Casement's role as emissary was self-appointed, and, as he was not a member of the IRB, that organisation's leadership wished to have one of their own contact Germany to negotiate German aid for an uprising the following year. He was seeking (but not limiting himself to) a shipment of arms. Casement, on the other hand, spent most of his energies recruiting Irish
prisoners of war in Germany to form a brigade to fight instead for Ireland. Some nationalists in Ireland saw this as a fruitless endeavour and preferred to seek weapons. Plunkett successfully got a promise of a German arms shipment to coincide with the Rising. According to
Ernest Blythe, Plunkett's republicanism did not prevent him from suggesting, at a briefing of Irish Volunteer organisers in January 1915, that in certain circumstances it would be in Irish interests for a German Catholic prince to be crowned
king of Ireland, nor did anyone present object. During the
Easter Rising, Plunkett and
Patrick Pearse argued in a conversation with
Desmond Fitzgerald that it would be beneficial for
Prince Joachim of Prussia to be crowned king. ==Easter Rising==