MacDonagh was the first teacher on the staff at St. Enda’s, the school he helped to found with Patrick Pearse. MacDonagh was well versed in literature, his enthusiasm and erudition earning him a position in the English Department at University College Dublin. His play 'When the Dawn is Come' was produced at the Abbey Theatre. He was appointed director of training for the Irish Volunteers in 1914, later joining the IRB. He was appointed to the IRB military committee in 1916. He was commander of the Second Battalion of Volunteers that occupied Jacob’s Biscuit Factory and surrounding houses during the Rising.
Ba é Mac Donnchadha an chéad mhúinteoir ar fhoireann Scoil Éanna, an scoil a bhunaigh sé in éineacht le Pádraig Mac Piarais. Bhí sé an-eolach ar chúrsaí litríochta, agus bhain a chuid dúthrachta agus léinn áit amach dó i roinn an Bhéarla sa Choláiste Ollscoile, Baile Átha Cliath. Léiríodh an dráma a scríobh sé, When the Dawn is Come, in Amharclann na Mainistreach. Ceapadh é ina stiúrthóir traenála ar Óglaigh na hÉireann in 1914, agus chuaigh sé isteach san IRB ina dhiaidh sin. Ceapadh Mac Donnchadha ar choiste míleata an IRB i 1916. Bhí sé ina cheannasaí ar 2a Cathlán na nÓglach, a ghlac seilbh ar Mhonarcha Brioscaí Jacob agus ar thithe sa chóngaracht le linn an Éirí Amach.