Every family has its myths. In mine, we were told that one of our antecedents had worked on the first maps of Ireland. As a child, I used to picture a solitary person in unspecified period dress – a tailcoat, perhaps some kind of cravat – striding pensively about fields and mountains, pen in hand. On summer holidays, I would stare out of the window of our red car as Donegal or Galway rolled by and wonder that such a task could be achieved. How did one man set about drawing a map of a whole country, of these towns and strands and trees and rivers?

All myths comprise a great deal of fanciful embroidery through which runs the distinct thread of truth: time and retelling will always refract reality. This mapper preyed on my mind. I thought about him, always, when I travelled around Ireland. I thought about him in my final year of school, when my geography exam required me to analyse a square of an unknown map. I wanted, as I often do, to know more, about his life, his work, who he had been and how he had mapped.

It took me a very long time to find him. What happened was this: a relative died and my parents were sent some items pertaining to the family. Among these was a hand-drawn map of an imaginary place, no larger than a hardback book, beautifully rendered in coloured inks. Also, an ancient photograph of a man seated in a doorway with a child on his knee. No tailcoats or cravats were present: this man wore a worn jacket and a low-brimmed hat, the dwelling behind him was a stone cottage with a latched half-door. The child gazed at the photographer with alert, inquisitive eyes.

Here they were, the mapper and his son, my great-great-grandfather with my great-grandfather on his knee. Returning to examine the map with a magnifying glass, I was able to see, in the highest left-hand corner, inside a tiny medallion, in what must have been painted with a minuscule brush, an intriguing tableau. A red-jacketed soldier was leaning to look into a theodolite mounted on a tripod; behind him, holding a measuring chain, stood a man instantly recognisable as the mapper from the photograph.

There was the same faded jacket, the turned-up hat, the beard, the rigidity of stance. Here he had been, hidden to the naked eye, for more than 150 years. This near-invisible yet politically freighted tableau transfixed me: the confident, possessive stance of the British soldier; the proximity and almost palpable anxiety of my great-great-grandfather behind him. So I went searching for whatever else I could find.

Locating him in written records proved difficult due to the ruling that Irish personnel for the Ordnance Survey were not permitted at this time to sign their own work: all survey notes and draft maps had to be certified and signed by a British army officer. I leafed through the immense Ordnance Survey archives in Dublin and found such fascinating documents as guidelines for typography and a letter from a landlady in Cavan who complained that the army sappers had damaged a bed. A command that all “labourers” be known only by the anglicised translation of their names. After a memorandum that “all Labourers be allowed one Week’s leave of absence”, was a list of signatures, including that of my great-great-grandfather. His name leapt off the page: a slanting script, not dissimilar to my own, written with a sure and deft hand.

It’s hard to describe the moment when I found this. I would have clapped my hands and cheered were it not for the hushed environs of the archives. Here he was and here it was: incontrovertable proof, in black and white, of the truth of what we were told as children. I wanted to turn to my fellow archive-perusers and say, you’ll never guess, just look at this.

When I read it for the second time, however, it was impossible not to notice that the letter was dated June 1853. Even someone with only the scantest grasp of history will know that Ireland suffered a cataclysmic great famine in the middle of the 19th century. Between 1846 and 1852, more than a million people died of starvation or famine-related disease; a further million were forced to emigrate, many of whom expired at sea; it’s worth noting that these numbers are considered by some historians as conservative estimates.

This memorandum showed that my great-great-grandfather was accompanying Ordnance Survey mapping divisions, acting as labourer and translator, making revisions in the aftermath of this disaster. He would have been traversing a country ravaged and laid waste: almost 30% of the population lost, whole villages wiped out, mass graves at the sides of roads, estates and fields reconfigured, unprecedented socio-political upheaval. It would have been his job to ensure that these grim changes were marked on the new post-famine versions of Ireland’s maps. What can that possibly have been like? How could a person who had lived through those times take on that task?

I have always taken issue with the edict, often aired in creative writing classes, that you should write what you know. Land is about a man, Tomás, and his family, striving to emerge from the long shadow of the great famine; it also tells the whole story of Ireland via one narrow scrap of land and all the people who have lived upon it.

For me, fiction comes from what you don’t know, from – in this case – the many things about the history of Ireland, particularly in the 19th century, that confounded me, that filled me with questions. Chief among these questions was how a disaster of such magnitude was permitted to happen so close to the centre of one of the richest empires in the world. Land, then, came from a sense of perplexity and outrage, spurred on by the handful of details I knew about my great-great-grandfather – and a hand-drawn map, a photograph, and a myth that turned out to contain more truth than we ever could have imagined.