Mac Diarmada was born John MacDermott in Corranmore, close to Kiltyclogher in County Leitrim, an area where the landscape was marked by reminders of poverty and oppression.
His father Donald McDermott was a member of the IRB and a friend of John Daly.

Surrounding Mac Diarmada in rural Leitrim, there were signs of Irish history throughout the area.
 There was an ancient sweat-house, Mass rocks from the penal times and the persecutions of the 17th and 18th centuries, and deserted abodes as an aftermath of the hunger of the 1840s.

He was educated by the Irish Christian Brothers.
In 1908 he moved to Dublin, by which time he already had a long involvement in several Irish separatist and cultural organisations, including Sinn Féin, the Irish Republican Brotherhood (IRB), the Ancient Order of Hibernians and the Gaelic League.
He was soon promoted to the Supreme Council of the IRB and eventually elected secretary.
He originally refused to join the IRB as it was condemned by the Catholic church but Bulmer Hobson convinced him otherwise.

In 1910 he became manager of the radical newspaper Irish Freedom, which he founded along with Bulmer Hobson and Denis McCullough.
He also became a national organiser for the IRB, and was taken under the wing of veteran Fenian Tom Clarke.
Indeed, over the year the two became nearly inseparable.
Shortly thereafter Mac Diarmada was stricken with polio and forced to walk with a cane.

In November 1913 Mac Diarmada was one of the original members of the Irish Volunteers, and continued to work to bring that organisation under IRB control.
In May 1915 Mac Diarmada was arrested in Tuam, County Galway, under the Defence of the Realm Act 1914 for giving a speech against enlisting into the British Army during the First World War.
Mac Diarmada was born John MacDermott in Corranmore, close to Kiltyclogher in County Leitrim, an area where the landscape was marked by reminders of poverty and oppression. His father Donald McDermott was a member of the IRB and a friend of John Daly.
Surrounding Mac Diarmada in rural Leitrim, there were signs of Irish history throughout the area.  There was an ancient sweat-house, Mass rocks from the penal times and the persecutions of the 17th and 18th centuries, and deserted abodes as an aftermath of the hunger of the 1840s.
He was educated by the Irish Christian Brothers. In 1908 he moved to Dublin, by which time he already had a long involvement in several Irish separatist and cultural organisations, including Sinn Féin, the Irish Republican Brotherhood (IRB), the Ancient Order of Hibernians and the Gaelic League. He was soon promoted to the Supreme Council of the IRB and eventually elected secretary. He originally refused to join the IRB as it was condemned by the Catholic church but Bulmer Hobson convinced him otherwise.
In 1910 he became manager of the radical newspaper Irish Freedom, which he founded along with Bulmer Hobson and Denis McCullough. He also became a national organiser for the IRB, and was taken under the wing of veteran Fenian Tom Clarke. Indeed, over the year the two became nearly inseparable. Shortly thereafter Mac Diarmada was stricken with polio and forced to walk with a cane.
In November 1913 Mac Diarmada was one of the original members of the Irish Volunteers, and continued to work to bring that organisation under IRB control. In May 1915 Mac Diarmada was arrested in Tuam, County Galway, under the Defence of the Realm Act 1914 for giving a speech against enlisting into the British Army during the First World War.
Seán Mac Diarmada