Michael O'Connor's collection of postcards, started by his father in 1895, is not just a valuable family heirloom but a remarkable record of social history.

Attics, garages, spare rooms, unopened cupboards, old trunks: these are the fabled places which fiction populates with forgotten treasure. And just occasionally, in real life, treasures do turn up in these places. When William O'Connor, from Clough, Co Laois, died in 1959, his postcard collection sat in the bottom of an old press in his family house for decades. In 1980, his son, Michael O'Connor, who was still living in the same house, pulled out the cards from the cupboard to entertain visiting American relatives during the archetypal rainy day. When they asked him, with dropped jaws, "What's the name of your postcard collection?" he realised he owned something special, and was an unwitting deltiologist - the formal name for one who collects postcards.

By the time of his death, William O'Connor had collected 3,000 postcards. He began his collection in 1895 at the age of seven.

"My father was the youngest of nine children and the year he was seven, three of his sisters and his only brother all emigrated to America. After a month, his brother disappeared from contact and was never heard from again. None of the three sisters ever came back to Ireland again, not even once. But they all sent back postcards to him, which is how his collection started."

William O'Connor brought his cards to school with him, and the other children brought in cards that they too had been sent from relatives in the US, and gave them to him. "1895 to 1930 was the golden age of postcards," O'Connor explains. "Everyone sent them; they became a craze, the text messages of the time. Then, with the Wall Street Crash, printers went out of business. People didn't have the price of a card or a stamp any longer."

There are now some 4,000 cards in the collection, since formally named the Chapel Hill Collection (the name of the Clough townland). About half of these are in albums spread out on the table before us in the living room of O'Connor's family home; the press where they lay untouched for two decades is still in the corner.

The 1,000-odd cards which have been added to the collection by his son since William O'Connor died are from the same period: the majority of the cards date from the 1890s through to the 1930s. Some are from Minnesota, where the sisters settled, but most are of Irish images. People knew that O'Connor collected cards, and thus his friends and relatives all sent him postcards - sometimes once a day - when they went on holiday. He also continued to be given cards of the period.

To look through a couple of albums is to get a flavour of the collection. There are four cards showing interiors and lifeboats of the Titanic; several portrait-style shots of the 1916 leaders and grainy but arresting images of a wrecked Dublin after the Rising; over a hundred St Patrick's Day cards; Easter cards; and scores of fairdays, and classic main-street rural-town shots. There are also lots of churches, convents and schools, including one of the "Chemical Laboratory, Cistercian College Roscrea" with medieval-looking instruments, and one of the "Noviatiate, Convent of Mercy, Carlow" which shows a room devoid of people, containing a large table, on which are lined up several wooden boxes, like old writing sets, which, one assumes, belonged to the novices and were to be their link with the outside world.

It's details like these which make cards of this era particularly interesting: one hand-tinted card depicting Lynch's Castle in Galway includes what looks like tramlines on the street, and women, who are all wearing the traditional red woollen skirts and crotcheted shawls.

"The content of a card is what makes it valuable, the record of social history," O'Connor explains. "Particularly ones that have people in them, so that clothing can be studied. Fair days are important too."

Due to the lack of lighting equipment, most photographs were taken outdoors; hence the frequent images of people gathered outside their cottages, peering doubtfully into the camera. There is one intriguing card, with the simple caption "Roadside butter market", depicting beshawled young girls outside a cottage, holding out baskets with immense lumps of butter within, looking exactly like the huge pieces of bog butter that have been found in recent times.

There are also hundreds of sentimental Ewardian cards in the collection, with subtle and not-so-subtle messages of love/endearment/ arm-chancing. One arm-chancer is of a couple kissing by a moonlit river, the woman on the man's lap, over a drawing of a mustard spoon, with the caption: "Oh meet me sweet love, by the light of the moon and let us enjoy a real hot mustard spoon."

He has never had the collection professionally valued, but O'Connor says that 20 years ago the Titanic Historical Society in Belfast offered him £20,000 sterling for his four rare Titanic cards.

One card, with a sepia image of Dooagh in Achill, is possibly the single most valuable card of all. This is not because of what is on the card, but the postmark on the back. "I had some of the cards once at Malahide at a collector's fair, and a stamp dealer asked if he could look at some of the stamps. When he saw the one of Dooagh, he got very excited - he took out his chequebook and said 'Name a price'."

The reason for the stamp dealer's excitement was the fact that the card had been posted in Keel in 1926, and had an English-language frank. According to the dealer, Keel post office opened in 1926 and was only in business six weeks before it burnt down. When it reopened, the frank was in Irish, making collectors items of whatever mail had passed through Keel's original and short-lived post office.

O'Connor has been invited to the US 11 times to show his cards, opportunities which, he happily admits, have given his life a new dimension. Apart from the family connection, the public interest in his collection has meant that he's never considered selling even a part of it.

Now retired, he says he wants to pass it on to his children, so keeping it in the family for another generation.