Whatever happened to the Famine? When the process of commemorating what is by common consent the most important event in modern Irish history began in 1995, there was a general expectation that Irish artists would find ways of marking the anniversary that went beyond speeches and ceremonies. There have indeed been impressive individual responses - Cathal Porteir's radio programmes on famine folklore, Garry Hynes's Abbey production of Tom Murphy's Famine, Patrick Cassidy's Famine suite, for instance. But it would be pushing things to claim that there has been a profound engagement with the legacy of the Great Hunger.

There are many reasons for this failure. Epic horrors tend to overwhelm the imagination. It is hard for artists to find a clear path between mere sensationalism on the one hand and the danger of aestheticising human suffering on the other. And there is the broader problem of the Famine's uneasy place in Irish intellectual life. There is still no single scholarly work offering a convincing and complex account of the events of 1844-1847 for the general reader. Many of the mechanisms of mass mortality - especially the ones by which some Irish Catholics gained from the Famine - remain virtually unknown. In this context, there is too much room for packaged self-pity, aimed more at claiming for Ireland the hall of fame award for Most Oppressed People Ever than at doing justice to the memory of the victims. Any artistic approach has to work its way, not through the layers of history, but through a miasma of sentimentality.

Alan O'Connor's photographic exhibition, Reminder - Famine Sites of Ireland, at the Guinness Hop Store in Dublin, is an important example of what can be done. It is a significant act of commemoration in itself, marking as it does the places in which people suffered and died. But it is also a powerful reminder of the kind of emotional discipline that the subject requires.

One of the reminders that is implicit in the exhibition is that there were many cameras in Ireland at the time of the Famine, but none was turned on the destitute and dying. Photography, then, was a pastime of the powerful, and it occurred to no one that it could be used to document an overwhelmingly visible disaster. And so the Famine, for us, belongs to the pre-modern world of sketches and lithographs. It exists in a time before our own. It is, inescapably, absent. In that sense, a photographic exhibition on The Famine seems a contradiction in terms. The victims died or emigrated. They were erased from Irish life. And how can we, by definition the descendants of the survivors, imagine those who did not survive? How can we place ourselves in relation to events that are so far from our own experiences? How can we picture what is not there?

Alan O'Connor has attempted to do just that, to photograph an absence. In a sense, what makes the exhibition so powerful is its near impossibility. Trying to represent in black-and-white documentary style photographs the events of 150 years is a crazy task. But the effort contains its own aesthetic - with so little to focus on, the camera has to pay attention to the details that historians cannot supply. What we see when we look first at his austere pictures are familiar things - fields, stones, ruins, buildings, soil, the traces of mankind on the face of the earth. But what we see when we look again is what is not there. There are no people. There is no society. There are only mute vestiges.

In his pictures of famine graves, the bones may be under the soil, but we do not see them. The tears may have stained the walls of the workhouses he has photographed, but they have long since evaporated. The refugees may have embarked from the now-rotting quay at Cobh, subject of another of his images, but their lives, their histories, and their descendants are elsewhere. The gun may have been used to shoot the landlord, but the finger has withered from the trigger. We are, as we stand in front of these images, in the presence of the dead.

No art can do justice to the suffering of the famine victims, and any attempt to do so runs the risk of bathos. But what is too monstrous to be adequately described can be evoked. The almost forensic clarity and rigour of these photographs is a matter not only of artistic, but also of moral restraint. They refuse to yield to our desire for self-pity. They refuse to offer the distractions of drama. They give us the barely visible remains of what happened. By giving to grief a local habitation and a name, they perform the most basic and the most powerful task of photography - to memorialise.

Photographs are, above all, evidence that something happened and that someone, at least, witnessed it. They are proof against forgetting. And what we remember when we look at these lingering traces of obliterated lives is the simple and astonishing fact that they were once here and that we ourselves live in their aftermath.

Reminder - Famine Sites Of Ireland is at the Guinness Hop Store, Dublin, until August 29th.