In Ireland in 1904, on leave from Africa from that year until 1905, Casement joined the Gaelic League, an organisation established in 1893 to preserve and revive the spoken and literary use of the Irish language.
He met the leaders of the powerful Irish Parliamentary Party (IPP) to lobby for his work in the Congo.
He did not support those, like the IPP, proposing Home Rule, as he felt sure the House of Lords would veto such efforts.
Casement was more impressed by Arthur Griffith's new Sinn Féin party (founded 1905), which called for an independent Ireland (through a non-violent series of strikes and boycotts) whose sole imperial tie would be a dual monarchy between Britain and Ireland, modeled on the policy example of Ferenc Deák in Hungary.
Casement joined the party in 1905.

Casement retired from the British consular service in the summer of 1913.
In November of that year he was one of those helping to form the Irish Volunteers.
He and Eoin MacNeill, later the organisation's chief of staff, co-wrote the Volunteers' manifesto.
In July 1914, Casement journeyed to the United States to promote and raise money for the Volunteers among the large and numerous Irish community there.
Through his friendship with men such as Bulmer Hobson, a member both of the Volunteers and of the secret Irish Republican Brotherhood (IRB), Casement established connections with exiled Irish nationalists, particularly Clan na Gael.

Elements of the suspicious Clan did not trust him completely, as he was not a member of the IRB and held views they considered too moderate, although others such as John Quinn regarded him as extreme.
Devoy, initially hostile to Casement for his part in conceding control of the Irish Volunteers to John Redmond, was won over in June, and another Clan leader, Joseph McGarrity, became devoted to Casement and remained so from then on.
The Howth gun-running in late July 1914, which Casement had helped to organise and finance, further enhanced his reputation.

In August 1914, at the outbreak of World War I, Casement and John Devoy arranged a meeting in New York with the western hemisphere's top-ranking German diplomat, Count Bernstorff, to propose a mutually beneficial plan: if Germany would sell guns to the Irish revolutionaries and provide military leaders, the Irish would revolt against England, diverting troops and attention from the war with Germany.
Bernstorff appeared sympathetic.
Casement and Devoy sent an envoy, Clan na Gael president John Kenny, to present their plan personally.
Kenny, while unable to meet the German Emperor, did receive a warm reception from Flotow, the German ambassador to Italy, and from Prince von Bülow.

In October 1914, Casement sailed for Germany via Norway, traveling in disguise and seeing himself as an ambassador of the Irish nation.
While the journey was his idea, Clan na Gael financed the expedition.
During their stop in Christiania, his companion Adler Christensen was taken to the British legation, where a reward was allegedly offered if Casement were "knocked on the head".
British diplomat Mansfeldt Findlay, in contrast, advised London that Christensen had "implied that their relations were of an unnatural nature and that consequently he had great power over this man".
No evidence was provided by Findlay for the insinuation.

Findlay's handwritten letter of 1914 is kept in University College, Dublin, and is viewable online.
This letter—written on official notepaper by Minister Findlay at the British Legation in Oslo—offers to Christensen the sum of £5,000 plus immunity from prosecution and free passage to the United States in return for information leading to the capture of Roger Casement.
That amount would be approximately £2,616,000 in 2014.


In November 1914, Casement negotiated a declaration by Germany which stated:
The Imperial Government formally declares that under no circumstances would Germany invade Ireland with a view to its conquest or the overthrow of any native institutions in that country.
Should the fortune of this Great War, that was not of Germany's seeking, ever bring in
In Ireland in 1904, on leave from Africa from that year until 1905, Casement joined the Gaelic League, an organisation established in 1893 to preserve and revive the spoken and literary use of the Irish language. He met the leaders of the powerful Irish Parliamentary Party (IPP) to lobby for his work in the Congo. He did not support those, like the IPP, proposing Home Rule, as he felt sure the House of Lords would veto such efforts. Casement was more impressed by Arthur Griffith's new Sinn Féin party (founded 1905), which called for an independent Ireland (through a non-violent series of strikes and boycotts) whose sole imperial tie would be a dual monarchy between Britain and Ireland, modeled on the policy example of Ferenc Deák in Hungary. Casement joined the party in 1905.
Casement retired from the British consular service in the summer of 1913. In November of that year he was one of those helping to form the Irish Volunteers. He and Eoin MacNeill, later the organisation's chief of staff, co-wrote the Volunteers' manifesto. In July 1914, Casement journeyed to the United States to promote and raise money for the Volunteers among the large and numerous Irish community there. Through his friendship with men such as Bulmer Hobson, a member both of the Volunteers and of the secret Irish Republican Brotherhood (IRB), Casement established connections with exiled Irish nationalists, particularly Clan na Gael.
Elements of the suspicious Clan did not trust him completely, as he was not a member of the IRB and held views they considered too moderate, although others such as John Quinn regarded him as extreme. Devoy, initially hostile to Casement for his part in conceding control of the Irish Volunteers to John Redmond, was won over in June, and another Clan leader, Joseph McGarrity, became devoted to Casement and remained so from then on. The Howth gun-running in late July 1914, which Casement had helped to organise and finance, further enhanced his reputation.
In August 1914, at the outbreak of World War I, Casement and John Devoy arranged a meeting in New York with the western hemisphere's top-ranking German diplomat, Count Bernstorff, to propose a mutually beneficial plan: if Germany would sell guns to the Irish revolutionaries and provide military leaders, the Irish would revolt against England, diverting troops and attention from the war with Germany. Bernstorff appeared sympathetic. Casement and Devoy sent an envoy, Clan na Gael president John Kenny, to present their plan personally. Kenny, while unable to meet the German Emperor, did receive a warm reception from Flotow, the German ambassador to Italy, and from Prince von Bülow.
In October 1914, Casement sailed for Germany via Norway, traveling in disguise and seeing himself as an ambassador of the Irish nation. While the journey was his idea, Clan na Gael financed the expedition. During their stop in Christiania, his companion Adler Christensen was taken to the British legation, where a reward was allegedly offered if Casement were "knocked on the head". British diplomat Mansfeldt Findlay, in contrast, advised London that Christensen had "implied that their relations were of an unnatural nature and that consequently he had great power over this man". No evidence was provided by Findlay for the insinuation.
Findlay's handwritten letter of 1914 is kept in University College, Dublin, and is viewable online. This letter—written on official notepaper by Minister Findlay at the British Legation in Oslo—offers to Christensen the sum of £5,000 plus immunity from prosecution and free passage to the United States in return for information leading to the capture of Roger Casement. That amount would be approximately £2,616,000 in 2014.

In November 1914, Casement negotiated a declaration by Germany which stated:
The Imperial Government formally declares that under no circumstances would Germany invade Ireland with a view to its conquest or the overthrow of any native institutions in that country. Should the fortune of this Great War, that was not of Germany's seeking, ever bring in 
Roger Casement