Most historians opt for 28 November 1905, as a founding date because it was on this date that Griffith first presented his 'Sinn Féin Policy'. In his writings, Griffith declared that the Act of Union of Great Britain and Ireland in 1800 was illegal and that, consequently, the Anglo-Irish dual monarchy which existed under Grattan's Parliament, and the so-called Constitution of 1782 was still in effect. Its first president was Edward Martyn.

The fundamental principles on which Sinn Féin was founded were outlined in an article published in 1904 by Griffith called the Resurrection of Hungary, in which, noting how in 1867 Hungary went from being part of the Austrian Empire to a separate co-equal kingdom in Austria-Hungary. Though not a monarchist himself, Griffith advocated such an approach for the Anglo-Irish relationship, namely that Ireland should become a separate kingdom alongside Great Britain, the two forming a dual monarchy with a shared monarch but separate governments, as it was thought this solution would be more palatable to the British. However, this idea was never really embraced by later separatist leaders, especially Michael Collins, and never came to anything, although Kevin O'Higgins toyed with the idea as a means of ending partition, shortly before his assassination in 1926.

Griffith sought to combine elements of Parnellism with the traditional separatist approach; he saw himself not as a leader but as providing a strategy which a new leader might follow. Central to his strategy was parliamentary abstention: the belief that Irish MPs should refuse to attend the Parliament of the United Kingdom at Westminster, but should instead establish a separate Irish parliament (with an administrative system based on local government) in Dublin.
In 1907 Sinn Féin unsuccessfully contested a by-election in North Leitrim, where the sitting MP, one Charles Dolan of Manorhamilton, County Leitrim, had defected to Sinn Féin. At this time Sinn Féin was being infiltrated by the Irish Republican Brotherhood, who saw it as a vehicle for their aims; it had several local councillors mostly in Dublin, including W. T. Cosgrave elected in 1909.