Auctioneers of historical artefacts, like anglers, are always hoping to land the big one: a chair Queen Victoria may have sat on, a gun fired by Michael Collins, an unused ticket for Titanic. It's not unusual for them to tell tall tales of "rare", "unique" and "newly discovered" antiques. But, occasionally, something truly remarkable turns up.

A recent call from Sheppard’s Irish Auction House, a family business in Co Laois, was such a case. “You might want to take a look at this,” said Philip Sheppard.

In a room behind displays of polished Georgian furniture, glass-cased Victorian taxidermy and oil paintings of long-forgotten aristocrats, he put a file on a baize-covered table. Sheppard's had been consigned an archive of unpublished letters that Jackie Kennedy had written to a trusted friend, both before and after she married John F Kennedy. They began when Jacqueline Lee Bouvier was a 21-year-old university student and ended when she was the world's most famous woman.

The letters – published in The Irish Times this week – gave detailed accounts of a privileged upbringing on the US east coast, trips to Europe by transatlantic liner, her love of books, meeting and falling in love with her future husband, becoming first lady, her children, and how she coped with the president's assassination, in 1963.

She wrote almost all of them on paper embossed with her various addresses, including the White House. They were for an Irish priest, Fr Joseph Leonard, then living in retirement in a seminary in Dublin, whom Kennedy first met during a visit to Dublin in 1950. She had been given his name as a contact by her step-aunt. Although, at 73, he was more than 50 years her senior, they struck up a deep friendship that lasted until his death, in 1964.

The letters, which are due to be auctioned on June 10th, are expected to sell for more than €1 million. But regardless of the outcome of the auction, their discovery is already changing perceptions of Jackie Kennedy Onassis.

Kennedy was intensely private and left no memoir. These letters reveal the woman behind the impeccably staged public persona. The personality that emerges is complex, witty, intelligent, well read, loyal, opinionated and certainly not the snob she is often painted as. There’s no hint of malice – although she makes some wickedly sharp observations about her mother-in-law, Rose Kennedy, and the duchess of Windsor, Wallis Simpson.

In the early letters she emerges as the epitome of 1950s American optimism, goodness, decency and, yes, innocence. By 1964 her dreams had been shattered.

Has any other woman met with such public Shakespeareian triumph and disaster? Yet she “treat[ed] those two impostors just the same” with dignity, grace and courage which would have delighted Kipling, one of her favourite childhood authors.

Many of the comments in her letters are instantly memorable. The most fascinating – “God will have a bit of explaining to do to me if I ever see him” – refers to the tragedy of her life after the president’s death.

Perhaps most poignant is her request to Fr Leonard to “Pray for us in 1963”. Little could she have dreamed of the horrors that lay ahead: the deaths of her son Patrick, who lived for just a couple of days after his birth, in August, and of her husband, in Dallas, on November 22nd, when she was 34.

In the 1960s Kennedy was not just first lady but also a major figure in Ireland. Irish on her mother’s side and Catholic, she was married to a man regarded here as a living saint. Her photograph often hung alongside those of JFK and the pope on kitchen walls from Donegal to Dingle.

Her heartbreaking final letters to Fr Leonard, revealing her efforts to make “peace with God” after the assassination, arrived in Dublin in early 1964. Four years later, believed to be desperate for security and privacy, Kennedy surprised the world by marrying the Greek shipping magnate Aristotle Onassis. Catholic Ireland appeared to feel let down that she had not remained a keening Widow Quin. The country fell out of love with her.

Kennedy was widowed again in 1975, when Onassis died. She lived subsequently in an apartment overlooking Central Park in New York, avoided publicity, and took a job as an editor with a publishing company. She contracted cancer in 1994 and died that year, at the age of 64.

We thought we knew everything there was to know about her life. Her letters to Fr Leonard show that we did not. Reading them – the last written exactly 50 years ago – was a pleasure and a privilege. This type of correspondence represents the end of the golden age of letter-writing, swept away by telephony, the internet and social media. What will future historians have: a few tweets?

If, as seems likely, the letters are eventually published in full, we might fall in love with Jackie Kennedy. Again.