Brother William Palladius Allen, better known as WP, joined the staff of the O'Connell School in North Richmond Street in the years before the Easter Rising, when many of those who later fought were still sitting in its classrooms.
In all, 134 of those "who went out" on Easter Week were past pupils of the Christian Brothers school, including three of those who were later executed, Éamonn Ceannt, Con Colbert and Seán Heuston, along with future taoiseach Seán Lemass.
When he joined O'Connell's staff, Allen, who was born in Newcestown, Co Cork, inherited a small museum dedicated to Ireland's industrial past.
A keen admirer of the Rising and those behind, he spent the next 70 years amassing an extraordinary trove of memorabilia.
By the time of his death in 1984 at the age of 92, he had gathered 13,000 different items.
Representing a mini-history of Ireland, it begins with stone axes from the Neolithic period and includes a printed proclamation from the Williamite era.
In addition, Brother Allen acquired Robert Emmet’s death mask, a prayer book owned by Daniel O’Connell, a full set of ration cards given out during the Famine, along with thousands of books, letters and pamphlets.
However, it is the memorabilia gathered from the Rising and, subsequently, the War of Independence and the Civil War that will now most interest historians, when it is opened to the public by the Military Archives in Cathal Brugha Barracks.
Many of those who were involved in the independence struggle were happy to gift items to Brother Allen; especially those on the anti-Treaty side who were loathe to donate anything to the archives of the State.
The most important item in the collection is undoubtedly Patrick Pearse's last letter before he was executed. Dated May 3rd, 1916, the yellowing piece of paper is franked with the name of Kilmainham Jail. Pearse was shot at dawn that day.
It is addressed to his mother and is one of the most quoted letters in Irish history. “It is the death I should have asked for if God had given me the choice of all deaths – to die a soldier’s death for Ireland and for freedom.
“We have done right. People will say hard things of us now, but later on they will praise us,” he wrote.
"It is such an iconic object," said military archivist Hugh Beckett, "especially when you know he is about to be executed. He has written it knowing that people will remember it afterwards."
The Allen archive includes full and half copies of the Proclamation; a holster that Michael Collins may have had on him at Béal na Bláth; a pair of infant booties stitched by Kathleen Clarke while in Holloway Prison and a match ticket from the Bloody Sunday game in Croke Park between Tipperary and Dublin in 1920.
The collection also has a cigarette taken from the pocket of Willie Pearse after his execution and features rare photographs of prominent figures in the revolution before the Civil War broke out.
One particularly rare photograph features General Seán Mac Eoin, Seán Moylan, Eoin O'Duffy, Liam Lynch, Gearóid O'Sullivan and Liam Mellows together smiling for the camera in the spring of 1922. Within a year both Lynch and Mellows, who were anti-Treaty, had been killed.
Brother Allen spent his final years cataloguing his collection for posterity, but he left nothing behind about himself . He was, said the former director of the National Museum Dr Anthony Lucas, a modest man.
Looking on the huge collection gathered by the Christian Brother, Beckett says: “It was such a coup for us to get it. We have the Bureau of Military History material here, but this is another level.”