This article examines the involvement of Peadar Ó Maicín (1878–1916) in the Socialist Party of Ireland/Cumannacht na hÉireann from 1909. It discusses the part played by the Irish language in Ó Maicín’s initial development of a class consciousness; its role in finally converting him to socialism; and his pivotal part in ensuring that the name, programme, and propaganda of the new party would be in Irish as well as English. It explores his insistence that socialism would benefit the revival of Irish, and that championing that revival would help to root the left itself in the specific realities of Irish society. International comparison is made with another Celtic socialist, before a disillusionment with the Irish left on Ó Maicín’s part is traced. The experience of this self-taught socialist and scholar sheds light on the neglected linguistic aspect of the Irish left in his time.