A native of Tipperary, MacDonagh trained as a priest but like his parents became a teacher, and was on the staff at St Enda’s, the school he helped to found with Padraig Pearse. A gifted poet, writer and dramatist, in 1909 he was a founding member of the Association of Secondary Teachers of Ireland and also was active in setting up the Irish Women’s Franchise League in 1911 which promoted Irish nationalism and the cultural revival.
He joined the Irish Volunteers in November 1913, becoming a member of the provisional committee and taking part in the Howth gun-running. MacDonagh believed Irish freedom would be achieved by what he called “zealous martyrs”, hopefully through peace but, if necessary, by war.

Although a member of the IRB from April 1915, he was not co-opted to the Military Council until early April 1916, and so had little part in planning the Rising. He is believed, however, to have contributed to the content of the Proclamation.

As one of the four Dublin battalion commandants, MacDonagh was in charge at Jacob’s biscuit factory in Bishop Street. His two most senior officers were Major John MacBride and Michael O’Hanrahan. Survived by his wife Muriel Gifford and his children Donagh and Barbara, MacDonagh was executed by firing squad at Kilmainham Jail on May 3.