Following his release in September 1915, he joined the secret Military Committee of the IRB, which was responsible for planning the rising.
Indeed, Mac Diarmada and Clarke were the people most responsible for it.
Mac Diarmada, like the rest of the signatories of the proclamation believed a "blood sacrifice" was necessary.
In 1914 he said "the Irish patriotic spirit will die forever unless a blood sacrifice is made in the next few years.”

Due to his disability, Mac Diarmada took little part in the fighting of Easter week, but was stationed at the headquarters in the General Post Office (GPO), as one of the Provisional Republican Government.
 Following the surrender, he nearly escaped execution by blending in with the large body of prisoners.
He was eventually recognised by Daniel Hoey of G Division.
Following a court-martial on 9 May, Mac Diarmada was executed by firing squad on 12 May at the age of 33.

In September 1919 Hoey was shot dead by Michael Collins's Squad.
Likewise, the British Officer Lee-Wilson, who ordered Mac Diarmada be shot, rather than imprisoned, was also killed in Cork on Collins's order during the Irish War of Independence.

Mac Diarmada had been in regular correspondence with Nell Ryan.
In his final letter he wrote: "Miss Ryan, she who in all probability, had I lived, would have been my wife". "
Min" Josephine Ryan and her sister, Phyllis, had been couriers to the GPO.
 They also visited Kilmainham Gaol, before his execution, and managed to evade arrest.
Min, a founder of Cumann na mBan, managed to escape from Ireland to America; she later married Richard Mulcahy.

Before his execution, Mac Diarmada wrote, "I feel happiness the like of which I have never experienced.
I die that the Irish nation might live!”
Following his release in September 1915, he joined the secret Military Committee of the IRB, which was responsible for planning the rising. Indeed, Mac Diarmada and Clarke were the people most responsible for it. Mac Diarmada, like the rest of the signatories of the proclamation believed a "blood sacrifice" was necessary. In 1914 he said "the Irish patriotic spirit will die forever unless a blood sacrifice is made in the next few years.”
Due to his disability, Mac Diarmada took little part in the fighting of Easter week, but was stationed at the headquarters in the General Post Office (GPO), as one of the Provisional Republican Government.  Following the surrender, he nearly escaped execution by blending in with the large body of prisoners. He was eventually recognised by Daniel Hoey of G Division. Following a court-martial on 9 May, Mac Diarmada was executed by firing squad on 12 May at the age of 33.
In September 1919 Hoey was shot dead by Michael Collins's Squad. Likewise, the British Officer Lee-Wilson, who ordered Mac Diarmada be shot, rather than imprisoned, was also killed in Cork on Collins's order during the Irish War of Independence. 
Mac Diarmada had been in regular correspondence with Nell Ryan. In his final letter he wrote: "Miss Ryan, she who in all probability, had I lived, would have been my wife". "Min" Josephine Ryan and her sister, Phyllis, had been couriers to the GPO.  They also visited Kilmainham Gaol, before his execution, and managed to evade arrest. Min, a founder of Cumann na mBan, managed to escape from Ireland to America; she later married Richard Mulcahy.
Before his execution, Mac Diarmada wrote, "I feel happiness the like of which I have never experienced. I die that the Irish nation might live!”
Seán Mac Diarmada