As a child the family moved to his mother's hometown of Draperstown, County Londonderry where his father worked for the railway line.

[1] It was while he was in Belfast that he became a member of Conradh na Gaeilge and started writing articles for the Peasant, Sinn Féin and An Claidheamh Soluis publications.

It also around this time, under the influence of Seán Mac Diarmada, Bulmer Hobson and Denis McCullough,[2] that he became a member of the secret Republican society the Irish Republican Brotherhood as well as operator of the Belfast branch of Na Fianna Éireann, the Irish Nationalist boy scout group.

Following his release, O'Shannon began to operate out of cities such as Dublin and Cork, where he worked to hold together the ITGWU in the wake of Connolly's death.

At the same time, Connolly's death had caused a power struggle with the Socialist Party of Ireland which had been largely inactive since the outbreak of World War I. O'Shannon, William O'Brien and Sean O'Casey had come to form one wing of the party, while James' son Roddy Connolly and Seán McLoughlin formed another.

O'Shannon was becoming increasingly militant, now declaring himself "an Irish Bolshevik" and stating that "the soviet idea was the only one that would confer freedom on Ireland".

In 1920 O'Shannon began travelling to Britain to speak on behalf of the Irish Revolution, trying to win the support of the British Labour movement.

He also spoke out at the execution of anti-treaty prisoners such as Rory O'Connor and Liam Mellows following the Battle of the Four Courts, which he condemned as "the greatest crime...committed in Ireland within these last ten years".

Their son was Cathal O'Shannon who became a veteran Irish broadcaster and journalist and their daughters were Gráinne and Finola, who were both involved in acting and film production.