Seán MacDiarmada was born in 1884, in Corranmore, Kiltyclogher, county Leitrim.
Seán MacDiarmada was an ardent nationalist, supporting many organisations that promoted Irish language and culture. He joined the Gaelic League and the Irish Catholic fraternity, the Ancient Order of Hibernians. He was national organiser for Sinn Féin, and became manager of the Irish Republican Brotherhood newspaper Irish Freedom in 1910. He was a close friend of the Republican, Tom Clark. He was one of the first members of the Irish Volunteers in 1913, serving on the provisional committee of the Volunteers and on the military committee of the Irish Republican Brotherhood, working to bring the two organizations together.
In 1915, Seán MacDiarmada was arrested in Tuam, County Galway for giving a speech against enlisting into the British Army, who were recruiting heavily for troops to fight abroad during WW1. Released a few months later, he joined the secret Military Committee of the I.R.B. and with Tom Clarke, began to organize the armed rebellion of Easter 1916.
Seán MacDiarmada was a member of the symbolic Provisional Republican Government and a signatory of the Proclamation of the Irish Republic.  He was stationed at the headquarters in the General Post Office during the insurrection of 1916, though weakness from recently contracted polio meant that he was not actively engaged himself.
Following the surrender of the G.P.O. he attempted to get away by blending in with the crowd of arrested men, but was recognised by Daniel Hoey, an Irishman working for the British in their headquarters at Dublin Castle (a "G. man" from G Division) Seán MacDiarmada was convicted by court-martial and executed by firing squad on the 12th May 1916 at the age of 33. Daniel Hoey was assasinated by Michael Collins's Squad, and Lee-Wilson, the British Officer who ordered Seán MacDiarmada's execution, was killed in Cork on the order of Michael Collins.