MacDiarmada emigrated to Glasgow in 1900, and from there to Belfast in 1902. A member of Conradh na Gaeilge, he was acquainted with Belfast nationalist and activist, Bulmer Hobson. He joined the Irish Republican Brotherhood in 1906 while still in Belfast, later transferring to Dublin in 1908 where he assumed managerial responsibility for the IRB newspaper Irish Freedom in 1910. He was appointed to the provisional committee of Irish Volunteers in 1913, and was subsequently drafted onto the military committee of the IRB in 1915. During the Rising MacDiarmada served in the GPO.
Chuaigh Mac Diarmada ar imirce go Glaschú i 1900 agus uaidh sin go Béal Feirste i 1902. Agus é mar bhall de Chonradh na Gaeilge bhí aithne aige ar Bulmer Hobson, náisiúnaí agus gníomhaí ó Bhéal Feirste. Chuaigh sé isteach i mBráithreachas Phoblacht na hÉireann (IRB) i 1906 agus é ina chónaí i mBéal Feirste. Bhog sé go Baile Átha Cliath i 1908, áit ar ghlac sé freagracht bhainistíochta ar nuachtán an IRB, Saoirse na hÉireann, sa bhliain 1910. Ceapadh é mar chomhalta ar Choiste Sealadach Óglaigh na hÉireann i 1913, agus tugadh isteach é mar chomhalta de choiste míleata an IRB i 1915. Le linn an Éirí Amach bhí Mac Diarmada in Ard-Oifig an Phoist.