Island Theatre Company's new play is about an old man losing his memory, and a young man who sells memory. Playwright Mike Finn writes about how what we forget shapes us as much as what we remember

Towards the end of his long life, Mark Twain remarked that when he was younger his mind was so active he could remember everything, whether it had happened or not. Noting his fading faculties, he lamented the fact that soon he would remember only the things that had never happened. Remembering things that never happened is as much a part of everyday life as forgetting the things that did. As Stewart Parker reminds us in his play Northern Star, the trouble is not that we forget some things, but that we misremember everything.

One of my earliest memories, or perhaps "mis-memories", is of a holiday spent in Butlin's in mortal dread of being exposed as an evil duck murderer. After breakfast one day, I left the dining hall clutching a bread roll in my five-year-old hand. No sooner had I emerged, blinking, into the morning downpour, than the bread roll was snatched from my grasp by a large duck. The amphibian in question waddled off into the distance, leaving me to reclaim the dignity which inevitably ebbs when one's breakfast is snaffled by someone lower down the food chain. Nonetheless, I achieved the kind of instant recovery that only five-year-olds can and by lunchtime had erased the whole sorry affair from my memory.

Erased it, that is, until a stern voice from the tannoy system announced to all and sundry that the Camp Commandant took a very dim view of people feeding his poultry and was this very day mourning the loss of one of his flock, who had choked on a bread roll administered by a person or persons unknown. Recognising myself as this person or persons unknown, I spent the rest of the holiday dodging the Redcoats who, I was convinced, had one purpose in life - to bring to justice the Butlin's duck slayer.

At least, that's how I remember it. My mother doesn't remember it at all. Sure, she remembers Butlin's, breakfasts, bread rolls, ducks, Redcoats, tannoys and me. But of ducks waddling off this mortal coil, she remembers nothing. Perhaps the image of her son as baby-faced duck choker was too much for her and she blotted it from her memory. Perhaps it just slipped her mind. Perhaps fond recollections of my subsequent, nonfatal, achievements have replaced it in her memory banks. Or perhaps it never happened.

Either way, whether real or imagined, the duck incident is lodged in my memory, as immovable as the fatal chunk of bread which proved the poor bird's downfall.

Friedrich Nietzsche wrote that it was "through the power of using the past for living and making history out of what has happened, does a person first become a person". In other words, whether we like it or not, we are what we remember, ducks and all. We are the sum total of what has gone before or, at least, what we recall of what has gone before. The past, constantly remembered and retold, reshaped and recas, provides the narratives with which we negotiate the world. Memories personal, familial, communal, social, national and universal all help to fill in the map which guides us towards the future, what we forget proving every bit as important as what we remember.

In 1997, with the 21st century imminent, I set about writing a play which would act as a sort of memorial to the previous century in my native Limerick. Pigtown became a story of many stories culled from the collective memory of a city. Its central character, Tommy Clocks, invites its audience to his funeral only to remember, as he lies in state, that he isn't actually dead. For the remainder of the evening, Tommy brings his new-found friends on a tour of his city's past where he finds enough of himself to return to his unfinished funeral. From this mixture of fact, fiction, things half remembered, half forgotten, misremembered and completely made up, some kind of universal truth somehow managed, despite the odds, to manifest itself. Audiences, to my eternal amazement, responded warmly to this collective act of re-remembering, revelling in the discovery that we are all products of time, place and most especially, memory.

In a retirement home in Iowa, a couple of years later, I was invited to talk to the residents about my play, memory, storytelling and the importance of community narrative. After doing my schtick, I was regaled for well over an hour with the memories of my elderly audience whose recollections of 20th century Iowa bore a remarkable similarity to the tales of 20th century Limerick I had woven into Pigtown. In swapping memories we found that our two towns, so different on the surface, were very similar underneath.

As a writer preoccupied with memory, I became intrigued by the consequences of memory loss, what we forget, shaping us every bit as much as what we remember. If we are the sum total of what we recall, what becomes of us when we begin to forget? "Old age", wrote Roger Bacon, "is the home of forgetfulness", but what if the forgetfulness is all there is?

In November 1901, a woman known simply as Frau Auguste D was brought to a clinic in Frankfurt suffering from inexplicable forgetfulness.

Exhibiting signs of extreme disorientation, reduced comprehension, paranoia and with a short-term memory so inhibited that she could no longer write her own name, Auguste threw down her pen and told her doctor: "I have lost myself." In losing herself and her name, Auguste helped find a name for her disease.

They called it after her doctor. His name was Alois Alzheimer.

In A Quiet Moment, a play I began writing in Iowa two years ago, Peter Conway, a purveyor of computer memory, returns from California to visit his father, Eddie, who is getting on in years and beginning to forget. Without a future and unable to find each other in the present, they plumb their shared memories, looking for that elusive point of contact which will confirm them, once again, as father and son.

Add to this mix a caring daughter, whose memories are of an absent brother, and Eddie's sidekick, who is more lost than anyone, and the result is A Quiet Moment. Only time will tell how memorable it will be. As is the nature of our business, the whole thing might be instantly forgotten. While that would be far from my intention, I would console myself with the thought that we are shaped as much by that which is forgotten as by that which is remembered.

Whatever happens, I'm sure I'll remember it differently anyway. Duck, anyone?

A Quiet Moment previews at the Belltable in Limerick tomorrow and Wednesday, and opens on Thursday, running until November 16th. It will tour in the new year.