MacDonagh was assistant headmaster at St. Enda's School, Scoil Éanna, and lecturer in English at University College Dublin.

He grew up in a household filled with music, poetry and learning and was instilled with a love of both English and Irish culture from a young age.

[citation needed] His brothers included future Sinn Féin politician, Joseph, and film director John.

MacDonagh was essential to the school's early success, on his marriage he took the position of lecturer in English at the National University, while continuing to support St Enda's.

MacDonagh remained devoted to the Irish language, and in 1910 he became tutor to a younger member of the Gaelic League, Joseph Plunkett.

On 3 January 1912 he married Muriel Gifford[6] (a member of the Church of Ireland, though neither she nor he was a churchgoer); their son, Donagh, was born that November, and their daughter, Barbara, in March 1915.

[7] He supported the strikers during the Dublin lockout and was a member of the "Industrial Peace Committee" alongside Joseph Plunkett, whose stated aim was to achieve a fair outcome to the dispute.

Around this time Tom Clarke asked him to plan the grandiose funeral of Jeremiah O'Donovan Rossa, which was a resounding propaganda success, largely due to the graveside oration delivered by Pearse.

Still a relative newcomer to the IRB, men such as Clarke may have been hesitant to elevate him to such a high position too soon, which raises the question as to why he should be admitted at all.

[9] As it was, despite MacDonagh's rank and the fact that he commanded one of the strongest battalions, they saw little fighting, as the British Army avoided the factory as they established positions in central Dublin.

[10] His widow, Muriel, died of heart failure while swimming in Skerries, County Dublin on 9 July 1917; his son Donagh MacDonagh became a judge, and was also a prominent poet, Broadway playwright,[11] songwriter and broadcaster, a central member of the Irish literary revival of the 1940s/1960s.

Yeats most famous nationalist poem Easter 1916 makes an allusion to MacDonagh as a friend of Pearse: "This other his helper and friend/ Was coming into his force/ He might have won fame in the end/ So sensitive his nature seemed/ So daring and sweet his thought".

In a poem rich with allegory – the Dark Cow (Irish: Bó Orann) is an 18th-century symbol of Ireland, for instance – Ledwidge wrote: 'He shall not hear the bittern cry In the wild sky, where he is lain, Nor voices of the sweeter birds Above the wailing of the rain Nor shall he know when loud March blows Thro’ slanting snows her fanfare shrill, Blowing to flame the golden cup Of many an upset daffodil.

But when the Dark Cow leaves the moor And pastures poor with greedy weeds, Perhaps he'll hear her low at morn, Lifting her horn in pleasant meads.