In 1901, the death of William Rooney prompted Kearney to join the Willie Rooney Branch of the Gaelic League. He joined the Irish Republican Brotherhood in 1903. He taught night classes in Irish and numbered Seán O'Casey among his pupils. He found work with the National Theatre Society and in 1904 was one of the first to inspect the derelict building that became the Abbey Theatre, which opened its doors on 27 December of that year. He assisted with props and performed occasional walk-on parts at the Abbey until 1916.