He joined the Irish Volunteers in 1913 and his family's house at Kimmage, south of Dublin, was used as a clearing station for arms imported in 1914. Sometime in 1915 Joseph 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. Plunkett kept a coded diary of his time in Germany, and one can follow his movements with accuracy during this time. 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. Plunkett 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 the Irish Brigade. Plunkett successfully got a promise of a German arms shipment to coincide with the rising.
1915 Mar 17. Plunkett leaves Ireland with a "verbal message" for Casement from those in command of the Irish Volunteers
1915 Apr 9. Plunkett reaches Berne and sees German Embassy, who wire Berlin for instructions "Joseph Mary Plunkett from Dublin, identified through British passport of March 16, member of the Committee of Irish Volunteers there, is authorised by that committee to convey important, only oral instructions to Sir Roger Casement, whom he assumes to be in Berlin, has asked for passage papers to Berlin. Request if necessary, address for Casement."
1915 Apr 13. Casement writes to Count Wedel asking that Plunkett be granted the necessary permission to enter German and see him. He writes "he is the most likely person they would choose for a messenger as being an invalid he could go to Switzerland from Dublin without exciting suspicion." At a young age Plunkett had been stricken with tuberculosis and spent part of his youth in the warmer climates of the Mediterranean and north Africa.
1915 Apr 14. Richard Myer and Capt Nadolny in Berlin agree Plunketts journey
1915 Apr 20. Plunkett finally arrives in Germany via France, Spain, Italy and Switzerland. Plunkett entered Germany on a US passport in the name of Joe Peters.
Just before Plunketts arrival, there appear to have only been 8 men in the Irish Brigade, so it was no wonder that Casement was despondant. These men were Keogh, Quinlisk, Kavanagh, Dowling, plus 4 more. Casement had drawn up a plan to return to USA with Keogh (Keogh being a US citizen). Dowling would go to Ireland, and the other 6 would be found employment in civilian jobs in Germany. On Plunkett's arrival, Keogh thought that things started to look more promising, with Plunkett taking over command of the recruiting. He did not interview men himself, that was done by Keogh and Dowling, but they interviewed individually each potential recruit outside the wire of the camp and got through about 100 men a day. If a man did decide to join, the Germans were asked to bring the man's kit out of the camp to the recruiting baracks, and the man then stayed there. Plunkett swore in about 50 men over a 2 week period.
Plunkett spent some 8 weeks actually in Germany. About three and a half weeks was spent at Limburg and the remainder in Berlin. In both places his diary shows that he spent a lot of time talking to Capt Boehm, the German sabotage man. He appears to have had meetings with Boehm on around 20 days and with Casement on 30 days of the 60 days he spent in Germany. In addition there were some 10 meetings with Meyer or Von Wedel of the German Foreign Office, and a few with Nadolny
From his arrival on 20th April until 6th May, Plunkett spent 2 weeks in Berlin talking with Casement on one hand,and the German Foreign Office and Sept IIIb on the other. He wrote a report for the Germans on the military situation in Ireland, the deployment of troops on a county by county basis.
1915 Apr 27. Quinlisk and Dowling meet Plunkett in Berlin. Quinlisk says of Plunkett "One would never associate the soldiers and burning patriot with the rather sickly looking young man, who looked more like a dreamer and a poet than anything else"
He then moved to Limburg and appears to have taken over the running of the recruitment campaign from Father Nicholson.
1915 May 7 . Plunkett arrives at Limburg, and meets Father Nicholson and Crotty. Plunkett appears to have stayed, while at Limburg, at the Order of Palatine Father's Monastery at Limburg. He stayed here until Jun 1, so just under half his time in Germany was spent at Limburg
1915 May 12. Plunkett writes to Casement "I believe that the chief reason of the small amount of influence that Father Nicholson has is due to the fact that they know he is here for a purpose" However the new interview methods used with the Irish prisoners appear ro be working and one can see from Plunkett's diary that they appear to have recruited 56 men between 8th and 12th May. Then for some reason they do not get any more men.
1915 May 18. Casement writes to Wedel that Plunkett will "stay on some time to arrange some matters he is interested in" In fact between 10th and 20th May Plunkett was having almost daily meetings with Boehm. They appear to have been negotiating guns, though it is not possible top be absolutely certain from Plunketts cryptic diary entries. Plunketts diary records figures against three of these days - 174,000; 10,000; 194,000. Then there is a time gap before figures resume with 25,000 plus big ships; 12,000 plus ship; 4,800 plus ship; 12,000
1915 June 1. Plunkett returns to Berlin. Over his entire stay he seems to have met with Boehm over 20 times, and Boehm was his main contact with the Germans.
1915 Jun 18. Casement accompanies Plunkett and Father Nicholson for Plunkett and Nicholson to say goodbye to the men. Nicholson was returning to New York, and Plunkett to Ireland. Plunkett, Boehm and Casement meet to discuss progress. Quinlisk says that Casement's position at this point was that "he was not wholly dissatisfied at the progress we had made, and in fact told us that the nucleus of the Brigade was all that he needed in order to convince the Imperial Government "
1915 Jun 19. Plunkett leaves German bound for Berne, Switzerland carrying a German passport issued to him by the German Foreign Office in the name of James Motley Pinkerton. He crossed the German Swiss border at Singen, and picked up his own papers there in the name of Plunkett.
He then travelled to New York to inform Clan na Gael leaders on the preparations for the revolt. On his return he was appointed to the IRB military council but fell ill in early 1916 and had to undergo throat surgery. Despite his illness, he took his place in the GPO and signed the Proclamation. Like the other signatories he was sentenced to death. He married the artist, Grace Gifford, in his cell in Kilmainham Gaol shortly before his execution on 4th May 1916 by firing squad.