Séan Mac Dermott was born in Leitrim in 1884 but emigrated to Glasgow in 1900, and in 1902 he moved to Belfast. He was interested in the Irish language and culture and joined a group called the Gaelic League. He joined the Irish Republican Brotherhood (I.R.B.) in 1906 and ran the I.R.B. newspaper Irish Freedom in 1910. Séan MacDiarmada got a disease called polio in 1912 but he was still appointed to high positions in the Irish Volunteers and in the I.R.B. Séan MacDiarmada fought in the G. P. O. during the 1916 Rising. He was executed on the 12th of May 1916.