An Irish-Ireland organization, founded in 1893, with the aim of "de-anglicizing" Ireland through the revival of Irish as a spoken language. By 1908, it had 600 branches throughout Ireland, though it remained "primarily an urban organization".
The League was originally non-political and non-sectarian. In 1915, however, the moderates under Douglas Hyde, its president, were ousted in a clandestine manoeuvre by the IRB; and the League declared that the political independence of Ireland was a primary aim. Most of the leaders of the Rising were members; indeed Patrick Pearse was the editor of its newspaper, An Claidheamh Soluis ("The Sword of Light").
The playwright Sean O'Casey was also a member. In his Autobiographies he remarks on the League's "genteel" membership. League members criticize O'Casey for his working-class attire – "Why don't you wear a collar and tie, Sean, and not come to the Branch with a muffler round your neck?" O'Casey complains in turn that the "nicely-suited, white-collared respectable members" of the League "knew nothing and cared less of the workers". And he sums them up as: "A lot of fretful popinjays lisping Irish wrongly." – Sean O'Casey, Drums Under the Window, 1946.
Religion played a large role in the League's revival of Irish. Many of the minor Catholic clergy were ardent supporters and its "increasingly politicized lower-middle-class Catholic membership discouraged Protestants from joining". Eoin MacNeill, its co-founder with Hyde (and later Chief-of-Staff of the Irish Volunteers), had no doubt that true religion and the native language were deeply interfused: "When we learn to speak Irish, we soon find that it is what we may call essential Irish to acknowledge God, His presence, and His help, even in our most trivial conversation."
Encyclopedia of Irish History and Culture: Gaelic League
Wikipedia: Gaelic League