The past is wildly unpredictable. The memory of the Easter Rising, the week-long rebellion in Dublin whose centenary is being marked this year, has long been haunted by an anxious question: is it over yet? The rising can be seen as a foundational event for three political entities: the Republic of Ireland, Northern Ireland and (though this is conveniently ignored) the current United Kingdom, which changed radically when most of Ireland won its independence. Yet the struggle has always been to decide whether it is history or current affairs, something that has happened or a harbinger of something yet to happen, done and dusted or unfinished business.

Contested legacy: echoes of the Easter Rising Read more

As a historical fact, the Rising seems quite small and self-contained. It was a little sideshow to the cataclysmic main event: the first world war. Even in Irish terms, it was, objectively, quite marginal. About 1,600 men and women took some part in the rebellion during Easter week of 1916. By contrast, about a quarter of a million Irishmen fought in the Great War. During the Rising 485 men, women and children (mostly civilians) died in Dublin. In the same week 570 Irish soldiers were killed in a single horrific German gas attack at Hulluch on the western front – an event that is scarcely remembered. The Rising is just a drop in an ocean of blood.

It was also staged by a minority of a minority. The rebels came from two fringe groups, the militantly nationalist Irish Volunteers and the tiny socialist Irish Citizen Army. It was planned and carried out by a faction within the secret conspiratorial Irish Republican Brotherhood. But even most of the militants who believed that an armed rebellion was a good idea in principle did not think that it made sense to go ahead in 1916 without public support and substantial assistance from the Germans.

And as a military adventure the Rising was, of course, a dismal failure. Very little happened outside Dublin, and in the capital the rising was largely a matter of holding a number of public buildings until the British authorities recovered from their shock and assembled the overwhelming force that inevitably crushed it. The Irish Republican Army that emerged after 1916 never attempted to replicate the tactics of Easter week, implicitly acknowledging that they were, from a practical point of view, useless.

So why is this relatively minor disturbance so potent? Precisely because it is so hard to say what it means. It is one of those events that has a protean quality – it continually changes its shape. The most famous lines about the Rising are WB Yeats’s from Easter 1916: All changed, changed utterly / A Terrible beauty is born. But in fact the terrible beauty was not just born: it remains alive. And, like any living thing, it alters over time. Among the things that change utterly and constantly is the meaning of the Rising itself.

Facebook Twitter Pinterest British soldiers inspect the shell of Dublin’s General Post Office after trhe fighting of 1916 Photograph: Hulton Archive/Getty Images

Easter Rising 100 years on: share your photos and stories Read more

This was true almost from the start. Many people knew exactly what they thought of it. Mainstream Irish nationalists saw it as tragic folly. Irish unionists, especially in Ulster, saw it as a stab in the back in Britain’s hour of need, final proof that Irish Catholics could never be trusted. But these simple meanings were challenged by much more complex emotions. When the British authorities executed 15 rebel leaders during May 1916 (a 16th, Sir Roger Casement, was hanged in London in August) the public mood began to change. The rebels, instead of being dangerous lunatics, became martyrs. More specifically they became Catholic martyrs. As the rebel leader Patrick Pearse had clearly envisaged, the sacrifice at Easter was folded into the greatest of all blood sacrifices, that of Christ himself.

What happened to the Rising is that it very quickly moved out of the realm of historical fact and into that of the imagination. On the level of fact, for example, the idea of blood sacrifice doesn’t really hold up. The rich archive of statements by rebels yields very few examples of young men and women who thought they were going out to be killed in a hopeless cause – most thought (naively) that they were going to win. And in fact the rebels suffered remarkably few casualties: just 16% of the dead of Easter week were rebels, and most of the casualties were poor slum dwellers caught in crossfire and in British shelling.

As Patrick Pearse envisaged, the sacrifice at Easter was folded into the greatest of blood sacrifices, that of Christ

These facts, though, scarcely matter. Imaginatively, an anonymous kid blown apart by a shell cannot compete with the heartbreaking drama of the great socialist James Connolly being executed by a firing squad while tied to a chair because he was already wounded and could not stand. When, on the 10th anniversary of the Rising in 1926, Seán O’Casey, another working-class socialist, tried to write the slum dwellers back into the story, his play The Plough and the Stars, was greeted by riots. The Rising itself, retrospectively reshaped as a ritual of heroic male self-sacrifice (the women were quickly written out too) is an even more potent drama than O’Casey’s masterpiece. It is gripping, compelling and intensely moving.

Yet this imaginative triumph is also the reason why the Rising generates such anxiety. Facts can be analysed, weighed, placed in perspective. They accommodate complexity and ambiguity. Imaginative happenings make different kinds of demands. They compel us to enter fully into their spirit. So what is the spirit of 1916? In part at least it is the spirit of armed propaganda, the notion that a visionary minority can, without popular consent, stage a symbolic coup that will awaken the slumbering majority to its historic duties. The problem with the Rising is that, even while the Irish state claims it as its foundation stone, its imaginative logic is equally available to violent minorities, such as the Provisional IRA and its even more marginal successors. Far beyond Ireland, the Rising can be drawn on by any armed group to prove that yesterday’s terrorists will be today’s martyrs and tomorrow’s triumphant revolutionaries.

How do we contain this imaginative power? In part by shifting back to facts. So far the commemorations of the Rising have been doing a pretty good job of restoring more complex realities, treating the rebels as neither saints nor terrorists but real political actors in a wider European conflict. It is encouraging that the biggest selling book on the centenary is not a hagiography but Joe Duffy’s painstaking recovery of the names and stories of the 40 children who were killed by rebels or British forces. The context of the first world war, the central role of women and Dublin’s horrendous poverty are all being written back into the story of the Rising.

Welcome as this is, however, facts will not be enough. We have to accept that the Rising will always be imaginatively powerful and to ask, as Yeats and O’Casey did, what exactly it is that we want to imagine. If the Rising is potent because it functions as much as a work of art as a historical event, the glory of art is that it can always be reimagined. The imaginative space that the Rising opens up can be filled with much more than myths of blood sacrifice. It can accommodate too the idea that the rebels themselves imagined: a real republic of equal citizens. That too is unfinished business that makes powerful demands on Ireland’s future.