Born in Leitrim, he emigrated to Glasgow in 1900 where he worked as a tram conductor, and from there came back to Belfast in 1902. A member of the Gaelic League, he was sworn into the IRB by Denis McCullough, and transferred to Dublin in 1908 where he managed the IRB newspaper Irish Freedom in 1910.
Although afflicted with polio in 1912, together with Tom Clarke, McCullough and Bulmer Hobson, McDermott (also known as Sean Mac Diarmada) is credited with revitalising the IRB, becoming a popular leader. After the outbreak of the First World War in August 1914, he campaigned against Irishmen joining the British army, and was jailed under the Defence of the Realm Act.
In a speech at Tralee, Co Kerry he claimed: “The Irish patriotic spirit will die forever unless a blood sacrifice is made in the next few years.” MacDiarmada was said to be obsessively secretive in his planning, excluding many of his fellow IRB men from the Rising conspiracy. A signatory of the Proclamation and a member of the Provisional Government, he spent the Rising in the GPO. MacDiarmada was tried by court-martial and executed by ﬁring squad at Kilmainham on May 12. He was unmarried.