Sean MacDiarmada was born in Leitrim and in his youth worked as a gardener and tram conductor. Later, in Belfast, he was sworn into the Irish Republican Brotherhood. In 1908 he was transferred to Dublin, where he developed a close personal and political friendship with Tom Clarke.

From this time onwards he was unrelenting in his organisation of the IRB and through his travels across Ireland became the movement's best-known and most popular personality. He was one of the founding members of the Irish Volunteers in 1913 and was Secretary of the Supreme Council of the IRB, member of the Military Council and member of the Provisional Government.