Seán was born in Leitrim, on a poor landscape that was littered with reminders of poverty and oppression, like mass rocks and deserted cottages. His father was a member of the IRB (Irish Republican Brotherhood) and a friend of John Daly, Republican leader.
By his twenties Seán was already extensively involved with Sinn Féin, the Gaelic League, and the IRB. He was one of the founding members of the Irish Volunteers and, with Thomas Clarke, was one of the main military planners of the Easter Rising.
After the Rising, MacDiarmuida was almost able to blend in with the other prisoners and so escape execution, but he was recognised by Daniel Hoey, who was later executed for this by Michael Collins’ Squad. In the end, MacDiarmuida was shot on May 12th. Before he died, he wrote,
“I feel happiness the like of which I have never experienced. I die that the Irish nation might live”.