The advent of WWI called to mind the old Irish saying, “England’s difficulty is Ireland’s opportunity.” Hoping to capitalize on England’s preoccupation with the war with Germany, the IRB decided the time had at last come to fight for Irish freedom.  Critical to the Irish nationalists’ plans was financial assistance from the Clan-na-Gael, led by the Philadelphia-based Irish nationalist, Joseph McGarrity. 
Along with providing financial support, the Clan helped connect Sir Roger Casement with the Germans, who promised to provide the Irish with arms and ammunition.  Casement also attempted to form an Irish Brigade among the Irish soldiers who had fought for England but were imprisoned in Germany; unfortunately, the attempt was mostly unsuccessful. 
“The Plot to Murder Casement,” The Gaelic American - Vol. XII No. 10, March 6, 1915, Whole Number 599. New York [N.Y.] : Gaelic American Pub. Co, 1915, 4. [Digital Library]
“Sir Roger Casement’s German Passport,” Vital Issue V. 2 No. 19. New York : F. J. L. Dorl, 1915, 5. [Digital Library]
Postcard, With Image of Officers of "The Irish Brigade", [n.d.]. [Digital Library]
McGarrity, Joseph. Postcard, With Fac-Simile of Agreement Signed By M. De C. Findlay, English Minister to Norway, n.d. [Digital Library]
Casement, Roger. Typescript Copy: "The Correct Copy of Roger Casement's Letter to Sir Edward Grey: 'A Black Chapter of English Perfidy,' February 1, 1915." 1915. [Digital Library]
"English Plot to Murder Sir Roger Casement Foiled,” The Gaelic American - Vol. XII No. 8, February 20, 1915, Whole Number 597. New York [N.Y.]: Gaelic American Pub. Co, 1915, 1, 5. [Digital Library]
In addition to Casement’s work with Germany were several groups that supported the plans for a Rising. When the National Volunteers were promised to help England fight Germany in the Great War, Irish nationalists separated and formed the Irish Volunteers.  One of the leaders of the Irish Volunteers was Eoin MacNeill, a professor and contributor to the Gaelic League. 
The Irish Citizen Army, led by James Connolly, was another organization that participated substantially in the Rising.  This group differed from the Irish Volunteers in its commitment to labor reform. 
O'Brien, Nora Connolly. The Irish Rebellion of 1916, or, The Unbroken Tradition. New York: Boni and Liveright, 1919. [Internet Archive]
Connolly, James. Socialism Made Easy in Two Sections ... By James Connolly, [Front Cover]. Chicago: Charles H. Kerr & Company Co-Operative, 1909. [Digital Library]
It was not just the men that served the Irish nation in the Rising, but also women, girls, and boys. Na Fianna Éireann was a boy scout nationalist group.  Women and girls joined the women’s nationalist association, the Cumann na mBan. 
McGarrity, Joseph. Typescript, Poem: "To the Fianna," By Joseph McGarrity, 1915. 1915. [Digital Library]
Photograph, Na Fianna Eireann Congress, 1913. 1913. [Digital Library]
Ryan, Desmond. Newspaper Clipping, “Twenty-Two Years After” The Irish Press, April 18, 1938. Philadelphia : The Irish Press, 1938, back. [Digital Library]
“Cumann na mBan Appeals to Irish-American Women," The Gaelic American - Vol. XII, No. 28, July 10, 1915, Whole Number 617. New York [N.Y.]: Gaelic American Pub. Co, 1915, 2. [Digital Library]
5. William J. Maloney, The Forged Casement Diaries (Dublin and Cork: The Talbot Press Limited, 1936), 109; Warre B. Wells and N. Marlowe, A History of the Irish Rebellion of 1916 (Lanham, MD: Ivan R. Dee, 2006), 111-114.
6. Charles Townshend, Easter 1916: The Irish Rebellion (Lanham, MD: Ivan R. Dee, 2006), 62-63, 413; John F. Boyle, The Irish Rebellion of 1916: A Brief History of the Revolt and Its Suppression (London: Constable and Company Limited, 1916), 39-40; Wells and Marlowe, A History of the Irish Rebellion of 1916, 49-50.
7. Boyle, The Irish Rebellion of 1916, 34; O’Donnell, America and the 1916 Rising, 9; Padraic Colum, “Formation of the Irish Volunteers and the Irish Citizen Army,” in Padraic Colum, et al., The Irish Rebellion of 1916 and its Martyrs: Erin’s Tragic Easter (New York: The Devin-Adair Company, 1916), 51.
9. BBC, “Profiles: The Irish Citizen Army,” Easter 1916: From Home Rule to Independence, September 24, 2014, http://www.bbc.co.uk/history/british/easterrising/profiles/po14.shtml; McGarry, The Rising, 63, 94.
Photograph, Joseph McGarrity, Standing With Chair. [Digital Library]